RFC3659.txt

Network Working Group                                         P. Hethmon
Request for Comments: 3659                              Hethmon Software
Updates: 959                                                  March 2007
Category: Standards Track


                           Extensions to FTP

Status of This Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The IETF Trust (2007).

Abstract

   This document specifies new FTP commands to obtain listings of remote
   directories in a defined format, and to permit restarts of
   interrupted data transfers in STREAM mode.  It allows character sets
   other than US-ASCII, and also defines an optional virtual file
   storage structure.
























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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Document Conventions . . . . . . . . . . . . . . . . . . . . .  3
       2.1.  Basic Tokens . . . . . . . . . . . . . . . . . . . . . .  4
       2.2.  Pathnames. . . . . . . . . . . . . . . . . . . . . . . .  4
       2.3.  Times. . . . . . . . . . . . . . . . . . . . . . . . . .  6
       2.4.  Server Replies . . . . . . . . . . . . . . . . . . . . .  7
       2.5.  Interpreting Examples. . . . . . . . . . . . . . . . . .  8
   3.  File Modification Time (MDTM). . . . . . . . . . . . . . . . .  8
       3.1.  Syntax . . . . . . . . . . . . . . . . . . . . . . . . .  9
       3.2.  Error Responses. . . . . . . . . . . . . . . . . . . . .  9
       3.3.  FEAT Response for MDTM . . . . . . . . . . . . . . . . . 10
       3.4.  MDTM Examples. . . . . . . . . . . . . . . . . . . . . . 10
   4.  File SIZE. . . . . . . . . . . . . . . . . . . . . . . . . . . 11
       4.1.  Syntax . . . . . . . . . . . . . . . . . . . . . . . . . 11
       4.2.  Error Responses. . . . . . . . . . . . . . . . . . . . . 12
       4.3.  FEAT Response for SIZE . . . . . . . . . . . . . . . . . 12
       4.4.  Size Examples. . . . . . . . . . . . . . . . . . . . . . 12
   5.  Restart of Interrupted Transfer (REST) . . . . . . . . . . . . 13
       5.1.  Restarting in STREAM Mode. . . . . . . . . . . . . . . . 14
       5.2.  Error Recovery and Restart . . . . . . . . . . . . . . . 14
       5.3.  Syntax . . . . . . . . . . . . . . . . . . . . . . . . . 15
       5.4.  FEAT Response for REST . . . . . . . . . . . . . . . . . 16
       5.5.  REST Example . . . . . . . . . . . . . . . . . . . . . . 17
   6.  A Trivial Virtual File Store (TVFS). . . . . . . . . . . . . . 17
       6.1.  TVFS File Names. . . . . . . . . . . . . . . . . . . . . 18
       6.2.  TVFS Pathnames . . . . . . . . . . . . . . . . . . . . . 18
       6.3.  FEAT Response for TVFS . . . . . . . . . . . . . . . . . 20
       6.4.  OPTS for TVFS. . . . . . . . . . . . . . . . . . . . . . 21
       6.5.  TVFS Examples. . . . . . . . . . . . . . . . . . . . . . 21
   7.  Listings for Machine Processing (MLST and MLSD). . . . . . . . 23
       7.1.  Format of MLSx Requests. . . . . . . . . . . . . . . . . 23
       7.2.  Format of MLSx Response. . . . . . . . . . . . . . . . . 24
       7.3.  File Name Encoding . . . . . . . . . . . . . . . . . . . 26
       7.4.  Format of Facts. . . . . . . . . . . . . . . . . . . . . 28
       7.5.  Standard Facts . . . . . . . . . . . . . . . . . . . . . 28
       7.6.  System Dependent and Local Facts . . . . . . . . . . . . 36
       7.7.  MLSx Examples. . . . . . . . . . . . . . . . . . . . . . 37
       7.8.  FEAT Response for MLSx . . . . . . . . . . . . . . . . . 49
       7.9.  OPTS Parameters for MLST . . . . . . . . . . . . . . . . 51
   8.  Impact on Other FTP Commands . . . . . . . . . . . . . . . . . 54
   9.  Character Sets and Internationalization. . . . . . . . . . . . 55
   10. IANA Considerations. . . . . . . . . . . . . . . . . . . . . . 55
       10.1. The OS Specific Fact Registry. . . . . . . . . . . . . . 56
       10.2. The OS Specific Filetype Registry. . . . . . . . . . . . 56





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   11. Security Considerations. . . . . . . . . . . . . . . . . . . . 57
   12. Normative References . . . . . . . . . . . . . . . . . . . . . 58
   Acknowledgments. . . . . . . . . . . . . . . . . . . . . . . . . . 59

1.  Introduction

   This document updates the File Transfer Protocol (FTP) [3].  Four new
   commands are added: "SIZE", "MDTM", "MLST", and "MLSD".  The existing
   command "REST" is modified.  Of those, the "SIZE" and "MDTM"
   commands, and the modifications to "REST" have been in wide use for
   many years.  The others are new.

   These commands allow a client to restart an interrupted transfer in
   transfer modes not previously supported in any documented way, and to
   obtain a directory listing in a machine friendly, predictable,
   format.

   An optional structure for the server's file store (NVFS) is also
   defined, allowing servers that support such a structure to convey
   that information to clients in a standard way, thus allowing clients
   more certainty in constructing and interpreting pathnames.

2.  Document Conventions

   This document makes use of the document conventions defined in BCP
   14, RFC 2119 [4].  That provides the interpretation of capitalized
   imperative words like MUST, SHOULD, etc.

   This document also uses notation defined in STD 9, RFC 959 [3].  In
   particular, the terms "reply", "user", "NVFS" (Network Virtual File
   System), "file", "pathname", "FTP commands", "DTP" (data transfer
   process), "user-FTP process", "user-PI" (user protocol interpreter),
   "user-DTP", "server-FTP process", "server-PI", "server-DTP", "mode",
   "type", "NVT" (Network Virtual Terminal), "control connection", "data
   connection", and "ASCII", are all used here as defined there.

   Syntax required is defined using the Augmented BNF defined in [5].
   Some general ABNF definitions that are required throughout the
   document will be defined later in this section.  At first reading, it
   may be wise to simply recall that these definitions exist here, and
   skip to the next section.










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2.1.  Basic Tokens

   This document imports the core ABNF definitions given in Appendix A
   of [5].  There definitions will be found for basic ABNF elements like
   ALPHA, DIGIT, SP, etc.  The following terms are added for use in this
   document.

      TCHAR          = VCHAR / SP / HTAB    ; visible plus white space
      RCHAR          = ALPHA / DIGIT / "," / "." / ":" / "!" /
                       "@" / "#" / "$" / "%" / "^" /
                       "&" / "(" / ")" / "-" / "_" /
                       "+" / "?" / "/" / "\" / "'" /
                       DQUOTE   ; <"> -- double quote character (%x22)
      SCHAR          = RCHAR / "=" ;

   The VCHAR (from [5]), RCHAR, SCHAR, and TCHAR types give basic
   character types from varying sub-sets of the ASCII character set for
   use in various commands and responses.

      token          = 1*RCHAR

   A "token" is a string whose precise meaning depends upon the context
   in which it is used.  In some cases it will be a value from a set of
   possible values maintained elsewhere.  In others it might be a string
   invented by one party to an FTP conversation from whatever sources it
   finds relevant.

   Note that in ABNF, string literals are case insensitive.  That
   convention is preserved in this document, and implies that FTP
   commands added by this specification have names that can be
   represented in any case.  That is, "MDTM" is the same as "mdtm",
   "Mdtm" and "MdTm" etc.  However note that ALPHA, in particular, is
   case sensitive.  That implies that a "token" is a case sensitive
   value.  That implication is correct, except where explicitly stated
   to the contrary in this document, or in some other specification that
   defines the values this document specifies be used in a particular
   context.

2.2.  Pathnames

   Various FTP commands take pathnames as arguments, or return pathnames
   in responses.  When the MLST command is supported, as indicated in
   the response to the FEAT command [6], pathnames are to be transferred
   in one of the following two formats.







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      pathname       = utf-8-name / raw
      utf-8-name     = <a UTF-8 encoded Unicode string>
      raw            = <any string that is not a valid UTF-8 encoding>

   Which format is used is at the option of the user-PI or server-PI
   sending the pathname.  UTF-8 encodings [2] contain enough internal
   structure that it is always, in practice, possible to determine
   whether a UTF-8 or raw encoding has been used, in those cases where
   it matters.  While it is useful for the user-PI to be able to
   correctly display a pathname received from the server-PI to the user,
   it is far more important for the user-PI to be able to retain and
   retransmit the identical pathname when required.  Implementations are
   advised against converting a UTF-8 pathname to a local charset that
   isn't capable of representing the full Unicode character repertoire,
   and then attempting to invert the charset translation later.  Note
   that ASCII is a subset of UTF-8.  See also [1].

   Unless otherwise specified, the pathname is terminated by the CRLF
   that terminates the FTP command, or by the CRLF that ends a reply.
   Any trailing spaces preceding that CRLF form part of the name.
   Exactly one space will precede the pathname and serve as a separator
   from the preceding syntax element.  Any additional spaces form part
   of the pathname.  See [7] for a fuller explanation of the character
   encoding issues.  All implementations supporting MLST MUST support
   [7].

   Note: for pathnames transferred over a data connection, there is no
   way to represent a pathname containing the characters CR and LF in
   sequence, and distinguish that from the end of line indication.
   Hence, pathnames containing the CRLF pair of characters cannot be
   transmitted over a data connection.  Data connections only contain
   file names transmitted from server-FTP to user-FTP as the result of
   one of the directory listing commands.  Files with names containing
   the CRLF sequence must either have that sequence converted to some
   other form, such that the other form can be recognised and be
   correctly converted back to CRLF, or be omitted from the listing.

   Implementations should also beware that the FTP control connection
   uses Telnet NVT conventions [8], and that the Telnet IAC character,
   if part of a pathname sent over the control connection, MUST be
   correctly escaped as defined by the Telnet protocol.

   NVT also distinguishes between CR, LF, and the end of line CRLF, and
   so would permit pathnames containing the pair of characters CR and LF
   to be correctly transmitted.  However, because such a sequence cannot
   be transmitted over a data connection (as part of the result of a
   LIST, NLST, or MLSD command), such pathnames are best avoided.




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   Implementors should also be aware that, although Telnet NVT
   conventions are used over the control connections, Telnet option
   negotiation MUST NOT be attempted.  See section 4.1.2.12 of [9].

2.2.1.  Pathname Syntax

   Except where TVFS is supported (see section 6), this specification
   imposes no syntax upon pathnames.  Nor does it restrict the character
   set from which pathnames are created.  This does not imply that the
   NVFS is required to make sense of all possible pathnames.  Server-PIs
   may restrict the syntax of valid pathnames in their NVFS in any
   manner appropriate to their implementation or underlying file system.
   Similarly, a server-PI may parse the pathname and assign meaning to
   the components detected.

2.2.2.  Wildcarding

   For the commands defined in this specification, all pathnames are to
   be treated literally.  That is, for a pathname given as a parameter
   to a command, the file whose name is identical to the pathname given
   is implied.  No characters from the pathname may be treated as
   special or "magic", thus no pattern matching (other than for exact
   equality) between the pathname given and the files present in the
   NVFS of the server-FTP is permitted.

   Clients that desire some form of pattern matching functionality must
   obtain a listing of the relevant directory, or directories, and
   implement their own file name selection procedures.

2.3.  Times

   The syntax of a time value is:

      time-val       = 14DIGIT [ "." 1*DIGIT ]

   The leading, mandatory, fourteen digits are to be interpreted as, in
   order from the leftmost, four digits giving the year, with a range of
   1000--9999, two digits giving the month of the year, with a range of
   01--12, two digits giving the day of the month, with a range of
   01--31, two digits giving the hour of the day, with a range of
   00--23, two digits giving minutes past the hour, with a range of
   00--59, and finally, two digits giving seconds past the minute, with
   a range of 00--60 (with 60 being used only at a leap second).  Years
   in the tenth century, and earlier, cannot be expressed.  This is not
   considered a serious defect of the protocol.






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   The optional digits, which are preceded by a period, give decimal
   fractions of a second.  These may be given to whatever precision is
   appropriate to the circumstance, however implementations MUST NOT add
   precision to time-vals where that precision does not exist in the
   underlying value being transmitted.

   Symbolically, a time-val may be viewed as

      YYYYMMDDHHMMSS.sss

   The "." and subsequent digits ("sss") are optional.  However the "."
   MUST NOT appear unless at least one following digit also appears.

   Time values are always represented in UTC (GMT), and in the Gregorian
   calendar regardless of what calendar may have been in use at the date
   and time indicated at the location of the server-PI.

   The technical differences among GMT, TAI, UTC, UT1, UT2, etc., are
   not considered here.  A server-FTP process should always use the same
   time reference, so the times it returns will be consistent.  Clients
   are not expected to be time synchronized with the server, so the
   possible difference in times that might be reported by the different
   time standards is not considered important.

2.4.  Server Replies

   Section 4.2 of [3] defines the format and meaning of replies by the
   server-PI to FTP commands from the user-PI.  Those reply conventions
   are used here without change.

      error-response = error-code SP *TCHAR CRLF
      error-code     = ("4" / "5") 2DIGIT

   Implementors should note that the ABNF syntax used in this document
   and in other FTP related documents (but not used in [3]), sometimes
   shows replies using the one-line format.  Unless otherwise explicitly
   stated, that is not intended to imply that multi-line responses are
   not permitted.  Implementors should assume that, unless stated to the
   contrary, any reply to any FTP command (including QUIT) may use the
   multi-line format described in [3].

   Throughout this document, replies will be identified by the three
   digit code that is their first element.  Thus the term "500 reply"
   means a reply from the server-PI using the three digit code "500".







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2.5.  Interpreting Examples

   In the examples of FTP dialogs presented in this document, lines that
   begin "C> " were sent over the control connection from the user-PI to
   the server-PI, lines that begin "S> " were sent over the control
   connection from the server-PI to the user-PI, and each sequence of
   lines that begin "D> " was sent from the server-PI to the user-PI
   over a data connection created just to send those lines and closed
   immediately after.  No examples here show data transferred over a
   data connection from the client to the server.  In all cases, the
   prefixes shown above, including the one space, have been added for
   the purposes of this document, and are not a part of the data
   exchanged between client and server.

3.  File Modification Time (MDTM)

   The FTP command, MODIFICATION TIME (MDTM), can be used to determine
   when a file in the server NVFS was last modified.  This command has
   existed in many FTP servers for many years, as an adjunct to the REST
   command for STREAM mode, thus is widely available.  However, where
   supported, the "modify" fact that can be provided in the result from
   the new MLST command is recommended as a superior alternative.

   When attempting to restart a RETRieve, the user-FTP can use the MDTM
   command or the "modify" fact to check if the modification time of the
   source file is more recent than the modification time of the
   partially transferred file.  If it is, then most likely the source
   file has changed, and it would be unsafe to restart the previously
   incomplete file transfer.

   Because the user- and server-FTPs' clocks are not necessarily
   synchronised, user-FTPs intending to use this method should usually
   obtain the modification time of the file from the server before the
   initial RETRieval, and compare that with the modification time before
   a RESTart.  If they differ, the files may have changed, and RESTart
   would be inadvisable.  Where this is not possible, the user-FTP
   should make sure to allow for possible clock skew when comparing
   times.

   When attempting to restart a STORe, the User FTP can use the MDTM
   command to discover the modification time of the partially
   transferred file.  If it is older than the modification time of the
   file that is about to be STORed, then most likely the source file has
   changed, and it would be unsafe to restart the file transfer.







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   Note that using MLST (described below), where available, can provide
   this information and much more, thus giving an even better indication
   that a file has changed and that restarting a transfer would not give
   valid results.

   Note that this is applicable to any RESTart attempt, regardless of
   the mode of the file transfer.

3.1. Syntax

   The syntax for the MDTM command is:

      mdtm          = "MdTm" SP pathname CRLF

   As with all FTP commands, the "MDTM" command label is interpreted in
   a case-insensitive manner.

   The "pathname" specifies an object in the NVFS that may be the object
   of a RETR command.  Attempts to query the modification time of files
   that exist but are unable to be retrieved may generate an error-
   response, or can result in a positive response carrying a time-val
   with an unspecified value, the choice being made by the server-PI.

   The server-PI will respond to the MDTM command with a 213 reply
   giving the last modification time of the file whose pathname was
   supplied, or a 550 reply if the file does not exist, the modification
   time is unavailable, or some other error has occurred.

      mdtm-response = "213" SP time-val CRLF /
                      error-response

   Note that when the 213 response is issued, that is, when there is no
   error, the format MUST be exactly as specified.  Multi-line responses
   are not permitted.

3.2.  Error Responses

   Where the command is correctly parsed but the modification time is
   not available, either because the pathname identifies no existing
   entity or because the information is not available for the entity
   named, then a 550 reply should be sent.  Where the command cannot be
   correctly parsed, a 500 or 501 reply should be sent, as specified in
   [3].  Various 4xy replies are also possible in appropriate
   circumstances.







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3.3.  FEAT Response for MDTM

   When replying to the FEAT command [6], a server-FTP process that
   supports the MDTM command MUST include a line containing the single
   word "MDTM".  This MAY be sent in upper or lower case or a mixture of
   both (it is case insensitive), but SHOULD be transmitted in upper
   case only.  That is, the response SHOULD be:

      C> Feat
      S> 211- <any descriptive text>
      S>  ...
      S>  MDTM
      S>  ...
      S> 211 End

   The ellipses indicate place holders where other features may be
   included, but are not required.  The one-space indentation of the
   feature lines is mandatory [6].

3.4.  MDTM Examples

   If we assume the existence of three files, A B and C, a directory D,
   two files with names that end with the string "ile6", and no other
   files at all, then the MDTM command may behave as indicated.  The
   "C>" lines are commands from user-PI to server-PI, the "S>" lines are
   server-PI replies.

      C> MDTM A
      S> 213 19980615100045.014
      C> MDTM B
      S> 213 19980615100045.014
      C> MDTM C
      S> 213 19980705132316
      C> MDTM D
      S> 550 D is not retrievable
      C> MDTM E
      S> 550 No file named "E"
      C> mdtm file6
      S> 213 19990929003355
      C> MdTm 19990929043300 File6
      S> 213 19991005213102
      C> MdTm 19990929043300 file6
      S> 550 19990929043300 file6: No such file or directory.

   From that we can conclude that both A and B were last modified at the
   same time (to the nearest millisecond), and that C was modified 20
   days and several hours later.




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   The times are in GMT, so file A was modified on the 15th of June,
   1998, at approximately 11am in London (summer time was then in
   effect), or perhaps at 8pm in Melbourne, Australia, or at 6am in New
   York.  All of those represent the same absolute time, of course.  The
   location where the file was modified, and consequently the local wall
   clock time at that location, is not available.

   There is no file named "E" in the current directory, but there are
   files named both "file6" and "19990929043300 File6".  The
   modification times of those files were obtained.  There is no file
   named "19990929043300 file6".

4.  File SIZE

   The FTP command, SIZE OF FILE (SIZE), is used to obtain the transfer
   size of a file from the server-FTP process.  This is the exact number
   of octets (8 bit bytes) that would be transmitted over the data
   connection should that file be transmitted.  This value will change
   depending on the current STRUcture, MODE, and TYPE of the data
   connection or of a data connection that would be created were one
   created now.  Thus, the result of the SIZE command is dependent on
   the currently established STRU, MODE, and TYPE parameters.

   The SIZE command returns how many octets would be transferred if the
   file were to be transferred using the current transfer structure,
   mode, and type.  This command is normally used in conjunction with
   the RESTART (REST) command when STORing a file to a remote server in
   STREAM mode, to determine the restart point.  The server-PI might
   need to read the partially transferred file, do any appropriate
   conversion, and count the number of octets that would be generated
   when sending the file in order to correctly respond to this command.
   Estimates of the file transfer size MUST NOT be returned; only
   precise information is acceptable.

4.1.  Syntax

   The syntax of the SIZE command is:

      size          = "Size" SP pathname CRLF

   The server-PI will respond to the SIZE command with a 213 reply
   giving the transfer size of the file whose pathname was supplied, or
   an error response if the file does not exist, the size is
   unavailable, or some other error has occurred.  The value returned is
   in a format suitable for use with the RESTART (REST) command for mode
   STREAM, provided the transfer mode and type are not altered.





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      size-response = "213" SP 1*DIGIT CRLF /
                      error-response

   Note that when the 213 response is issued, that is, when there is no
   error, the format MUST be exactly as specified.  Multi-line responses
   are not permitted.

4.2.  Error Responses

   Where the command is correctly parsed but the size is not available,
   perhaps because the pathname identifies no existing entity or because
   the entity named cannot be transferred in the current MODE and TYPE
   (or at all), then a 550 reply should be sent.  Where the command
   cannot be correctly parsed, a 500 or 501 reply should be sent, as
   specified in [3].  The presence of the 550 error response to a SIZE
   command MUST NOT be taken by the client as an indication that the
   file cannot be transferred in the current MODE and TYPE.  A server
   may generate this error for other reasons -- for instance if the
   processing overhead is considered too great.  Various 4xy replies are
   also possible in appropriate circumstances.

4.3.  FEAT Response for SIZE

   When replying to the FEAT command [6], a server-FTP process that
   supports the SIZE command MUST include a line containing the single
   word "SIZE".  This word is case insensitive, and MAY be sent in any
   mixture of upper or lower case, however it SHOULD be sent in upper
   case.  That is, the response SHOULD be:

      C> FEAT
      S> 211- <any descriptive text>
      S>  ...
      S>  SIZE
      S>  ...
      S> 211 END

   The ellipses indicate place holders where other features may be
   included, and are not required.  The one-space indentation of the
   feature lines is mandatory [6].

4.4.  Size Examples

   Consider a text file "Example" stored on a Unix(TM) server where each
   end of line is represented by a single octet.  Assume the file
   contains 112 lines, and 1830 octets total.  Then the SIZE command
   would produce:





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      C> TYPE I
      S> 200 Type set to I.
      C> size Example
      S> 213 1830
      C> TYPE A
      S> 200 Type set to A.
      C> Size Example
      S> 213 1942

   Notice that with TYPE=A the SIZE command reports an extra 112 octets.
   Those are the extra octets that need to be inserted, one at the end
   of each line, to provide correct end-of-line semantics for a transfer
   using TYPE=A.  Other systems might need to make other changes to the
   transfer format of files when converting between TYPEs and MODEs.
   The SIZE command takes all of that into account.

   Since calculating the size of a file with this degree of precision
   may take considerable effort on the part of the server-PI, user-PIs
   should not used this command unless this precision is essential (such
   as when about to restart an interrupted transfer).  For other uses,
   the "Size" fact of the MLST command (see section 7.5.7) ought be
   requested.

5.  Restart of Interrupted Transfer (REST)

   To avoid having to resend the entire file if the file is only
   partially transferred, both sides need some way to agree on where in
   the data stream to restart the data transfer.

   The FTP specification [3] includes three modes of data transfer,
   STREAM, Block, and Compressed.  In Block and Compressed modes, the
   data stream that is transferred over the data connection is
   formatted, allowing the embedding of restart markers into the stream.
   The sending DTP can include a restart marker with whatever
   information it needs to be able to restart a file transfer at that
   point.  The receiving DTP can keep a list of these restart markers,
   and correlate them with how the file is being saved.  To restart the
   file transfer, the receiver just sends back that last restart marker,
   and both sides know how to resume the data transfer.  Note that there
   are some flaws in the description of the restart mechanism in STD 9,
   RFC 959 [3].  See section 4.1.3.4 of RFC 1123 [9] for the
   corrections.









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5.1.  Restarting in STREAM Mode

   In STREAM mode, the data connection contains just a stream of
   unformatted octets of data.  Explicit restart markers thus cannot be
   inserted into the data stream, they would be indistinguishable from
   data.  For this reason, the FTP specification [3] did not provide the
   ability to do restarts in stream mode.  However, there is not really
   a need to have explicit restart markers in this case, as restart
   markers can be implied by the octet offset into the data stream.

   Because the data stream defines the file in STREAM mode, a different
   data stream would represent a different file.  Thus, an offset will
   always represent the same position within a file.  On the other hand,
   in other modes than STREAM, the same file can be transferred using
   quite different octet sequences and yet be reconstructed into the one
   identical file.  Thus an offset into the data stream in transfer
   modes other than STREAM would not give an unambiguous restart point.

   If the data representation TYPE is IMAGE and the STRUcture is File,
   for many systems the file will be stored exactly in the same format
   as it is sent across the data connection.  It is then usually very
   easy for the receiver to determine how much data was previously
   received, and notify the sender of the offset where the transfer
   should be restarted.  In other representation types and structures
   more effort will be required, but it remains always possible to
   determine the offset with finite, but perhaps non-negligible, effort.
   In the worst case, an FTP process may need to open a data connection
   to itself, set the appropriate transfer type and structure, and
   actually transmit the file, counting the transmitted octets.

   If the user-FTP process is intending to restart a retrieve, it will
   directly calculate the restart marker and send that information in
   the RESTart command.  However, if the user-FTP process is intending
   to restart sending the file, it needs to be able to determine how
   much data was previously sent, and correctly received and saved.  A
   new FTP command is needed to get this information.  This is the
   purpose of the SIZE command, as documented in section 4.

5.2.  Error Recovery and Restart

   STREAM mode transfers with FILE STRUcture may be restarted even
   though no restart marker has been transferred in addition to the data
   itself.  This is done by using the SIZE command, if needed, in
   combination with the RESTART (REST) command, and one of the standard
   file transfer commands.

   When using TYPE ASCII or IMAGE, the SIZE command will return the
   number of octets that would actually be transferred if the file were



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   to be sent between the two systems, i.e., with type IMAGE, the SIZE
   normally would be the number of octets in the file.  With type ASCII,
   the SIZE would be the number of octets in the file including any
   modifications required to satisfy the TYPE ASCII CR-LF end-of-line
   convention.

5.3.  Syntax

   The syntax for the REST command when the current transfer mode is
   STREAM is:

      rest          = "Rest" SP 1*DIGIT CRLF

   The numeric value gives the number of octets of the immediately-
   following transfer to not actually send, effectively causing the
   transmission to be restarted at a later point.  A value of zero
   effectively disables restart, causing the entire file to be
   transmitted.  The server-PI will respond to the REST command with a
   350 reply, indicating that the REST parameter has been saved, and
   that another command, which should be either RETR or STOR, should
   then follow to complete the restart.

      rest-response = "350" SP *TCHAR CRLF /
                      error-response

   Server-FTP processes may permit transfer commands other than RETR and
   STOR, such as APPE and STOU, to complete a restart; however, this is
   not recommended.  STOU (store unique) is undefined in this usage, as
   storing the remainder of a file into a unique file name is rarely
   going to be useful.  If APPE (append) is permitted, it MUST act
   identically to STOR when a restart marker has been set.  That is, in
   both cases, octets from the data connection are placed into the file
   at the location indicated by the restart marker value.

   The REST command is intended to complete a failed transfer.  Use with
   RETR is comparatively well defined in all cases, as the client bears
   the responsibility of merging the retrieved data with the partially
   retrieved file.  It may choose to use the data obtained other than to
   complete an earlier transfer, or to re-retrieve data that had been
   retrieved before.  With STOR, however, the server must insert the
   data into the file named.  The results are undefined if a client uses
   REST to do other than restart to complete a transfer of a file that
   had previously failed to completely transfer.  In particular, if the
   restart marker set with a REST command is not at the end of the data
   currently stored at the server, as reported by the server, or if
   insufficient data are provided in a STOR that follows a REST to
   extend the destination file to at least its previous size, then the
   effects are undefined.



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   The REST command must be the last command issued before the data
   transfer command that is to cause a restarted, rather than a
   complete, file transfer.  The effect of issuing a REST command at any
   other time is undefined.  The server-PI may react to a badly
   positioned REST command by issuing an error response to the following
   command, not being a restartable data transfer command, or it may
   save the restart value and apply it to the next data transfer
   command, or it may silently ignore the inappropriate restart attempt.
   Because of this, a user-PI that has issued a REST command, but that
   has not successfully transmitted the following data transfer command
   for any reason, should send another REST command before the next data
   transfer command.  If that transfer is not to be restarted, then
   "REST 0" should be issued.

   An error response will follow a REST command only when the server
   does not implement the command, or when the restart marker value is
   syntactically invalid for the current transfer mode (e.g., in STREAM
   mode, something other than one or more digits appears in the
   parameter to the REST command).  Any other errors, including such
   problems as restart marker out of range, should be reported when the
   following transfer command is issued.  Such errors will cause that
   transfer request to be rejected with an error indicating the invalid
   restart attempt.

5.4.  FEAT Response for REST

   Where a server-FTP process supports RESTart in STREAM mode, as
   specified here, it MUST include, in the response to the FEAT command
   [6], a line containing exactly the string "REST STREAM".  This string
   is not case sensitive, but it SHOULD be transmitted in upper case.
   Where REST is not supported at all or supported only in block or
   compressed modes, the REST line MUST NOT be included in the FEAT
   response.  Where required, the response SHOULD be:

      C> feat
      S> 211- <any descriptive text>
      S>  ...
      S>  REST STREAM
      S>  ...
      S> 211 end

   The ellipses indicate place holders where other features may be
   included, and are not required.  The one-space indentation of the
   feature lines is mandatory [6].







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5.5.  REST Example

   Assume that the transfer of a largish file has previously been
   interrupted after 802816 octets had been received, that the previous
   transfer was with TYPE=I, and that it has been verified that the file
   on the server has not since changed.

      C> TYPE I
      S> 200 Type set to I.
      C> PORT 127,0,0,1,15,107
      S> 200 PORT command successful.
      C> REST 802816
      S> 350 Restarting at 802816. Send STORE or RETRIEVE
      C> RETR cap60.pl198.tar
      S> 150 Opening BINARY mode data connection
      [...]
      S> 226 Transfer complete.

6.  A Trivial Virtual File Store (TVFS)

   Traditionally, FTP has placed almost no constraints upon the file
   store (NVFS) provided by a server.  This specification does not alter
   that.  However, it has become common for servers to attempt to
   provide at least file system naming conventions modeled loosely upon
   those of the UNIX(TM) file system.  This is a tree-structured file
   system, built of directories, each of which can contain other
   directories, or other kinds of files, or both.  Each file and
   directory has a name relative to the directory that contains it,
   except for the directory at the root of the tree, which is contained
   in no other directory, and hence has no name of its own.

   That which has so far been described is perfectly consistent with the
   standard FTP NVFS and access mechanisms.  The "CWD" command is used
   to move from one directory to an embedded directory.  "CDUP" may be
   provided to return to the parent directory, and the various file
   manipulation commands ("RETR", "STOR", the rename commands, etc.) are
   used to manipulate files within the current directory.

   However, it is often useful to be able to reference files other than
   by changing directories, especially as FTP provides no guaranteed
   mechanism to return to a previous directory.  The Trivial Virtual
   File Store (TVFS), if implemented, provides that mechanism.









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6.1.  TVFS File Names

   Where a server implements the TVFS, no elementary file name shall
   contain the character "/".  Where the underlying natural file store
   permits files, or directories, to contain the "/" character in their
   names, a server-PI implementing TVFS must encode that character in
   some manner whenever file or directory names are being returned to
   the user-PI, and reverse that encoding whenever such names are being
   accepted from the user-PI.

   The encoding method to be used is not specified here.  Where some
   other character is illegal in file and directory names in the
   underlying file store, a simple transliteration may be sufficient.
   Where there is no suitable substitute character a more complex
   encoding scheme, possibly using an escape character, is likely to be
   required.

   With the one exception of the unnamed root directory, a TVFS file
   name may not be empty.  That is, all other file names contain at
   least one character.

   With the sole exception of the "/" character, any valid IS10646
   character [10] may be used in a TVFS file name.  When transmitted,
   file name characters are encoded using the UTF-8 encoding [2].  Note
   that the two-character sequence CR LF occurring in a file name will
   make that name impossible to transmit over a data connection.
   Consequently, it should be avoided, or if that is impossible to
   achieve, it MUST be encoded in some reversible way.

6.2.  TVFS Pathnames

   A TVFS "Pathname" combines the file or directory name of a target
   file or directory, with the directory names of zero or more enclosing
   directories, so as to allow the target file or directory to be
   referenced other than when the server's "current working directory"
   is the directory directly containing the target file or directory.

   By definition, every TVFS file or directory name is also a TVFS
   pathname.  Such a pathname is valid to reference the file from the
   directory containing the name, that is, when that directory is the
   server-FTP's current working directory.

   Other TVFS pathnames are constructed by prefixing a pathname by a
   name of a directory from which the path is valid, and separating the
   two with the "/" character.  Such a pathname is valid to reference
   the file or directory from the directory containing the newly added
   directory name.




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   Where a pathname has been extended to the point where the directory
   added is the unnamed root directory, the pathname will begin with the
   "/" character.  Such a path is known as a fully qualified pathname.
   Fully qualified paths may, obviously, not be further extended, as, by
   definition, no directory contains the root directory.  Being unnamed,
   it cannot be represented in any other directory.  A fully qualified
   pathname is valid to reference the named file or directory from any
   location (that is, regardless of what the current working directory
   may be) in the virtual file store.

   Any pathname that is not a fully qualified pathname may be referred
   to as a "relative pathname" and will only correctly reference the
   intended file when the current working directory of the server-FTP is
   a directory from which the relative pathname is valid.

   As a special case, the pathname "/" is defined to be a fully
   qualified pathname referring to the root directory.  That is, the
   root directory does not have a directory (or file) name, but does
   have a pathname.  This special pathname may be used only as is as a
   reference to the root directory.  It may not be combined with other
   pathnames using the rules above, as doing so would lead to a pathname
   containing two consecutive "/" characters, which is an undefined
   sequence.

6.2.1.  Notes

   +  It is not required, or expected, that there be only one fully
      qualified pathname that will reference any particular file or
      directory.

   +  As a caveat, though the TVFS file store is basically tree
      structured, there is no requirement that any file or directory
      have only one parent directory.

   +  As defined, no TVFS pathname will ever contain two consecutive "/"
      characters.  Such a name is not illegal however, and may be
      defined by the server for any purpose that suits it.  Clients
      implementing this specification should not assume any semantics
      for such names.

   +  Similarly, other than the special case path that refers to the
      root directory, no TVFS pathname constructed as defined here will
      ever end with the "/" character.  Such names are also not illegal,
      but are undefined.

   +  While any legal IS10646 character is permitted to occur in a TVFS
      file or directory name, other than "/", server FTP implementations
      are not required to support all possible IS10646 characters.  The



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      subset supported is entirely at the discretion of the server.  The
      case (where it exists) of the characters that make up file,
      directory, and pathnames may be significant.  Unless determined
      otherwise by means unspecified here, clients should assume that
      all such names are comprised of characters whose case is
      significant.  Servers are free to treat case (or any other
      attribute) of a name as irrelevant, and hence map two names that
      appear to be distinct onto the same underlying file.

   +  There are no defined "magic" names, like ".", ".." or "C:".
      Servers may implement such names, with any semantics they choose,
      but are not required to do so.

   +  TVFS imposes no particular semantics or properties upon files,
      guarantees no access control schemes, or any of the other common
      properties of a file store.  Only the naming scheme is defined.

6.3.  FEAT Response for TVFS

   In response to the FEAT command [6] a server that wishes to indicate
   support for the TVFS as defined here will include a line that begins
   with the four characters "TVFS" (in any case, or mixture of cases,
   upper case is not required).  Servers SHOULD send upper case.

   Such a response to the FEAT command MUST NOT be returned unless the
   server implements TVFS as defined here.

   Later specifications may add to the TVFS definition.  Such additions
   should be notified by means of additional text appended to the TVFS
   feature line.  Such specifications, if any, will define the extra
   text.

   Until such a specification is defined, servers should not include
   anything after "TVFS" in the TVFS feature line.  Clients, however,
   should be prepared to deal with arbitrary text following the four
   defined characters, and simply ignore it if unrecognized.

   A typical response to the FEAT command issued by a server
   implementing only this specification would be:

      C> feat
      S> 211- <any descriptive text>
      S>  ...
      S>  TVFS
      S>  ...
      S> 211 end





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   The ellipses indicate place holders where other features may be
   included, but are not required.  The one-space indentation of the
   feature lines is mandatory [6] and is not counted as one of the first
   four characters for the purposes of this feature listing.

   The TVFS feature adds no new commands to the FTP command repertoire.

6.4.  OPTS for TVFS

   There are no options in this TVFS specification, and hence there is
   no OPTS command defined.

6.5.  TVFS Examples

   Assume a TVFS file store is comprised of a root directory, which
   contains two directories (A and B) and two non-directory files (X and
   Y).  The A directory contains two directories (C and D) and one other
   file (Z).  The B directory contains just two non-directory files (P
   and Q) and the C directory also two non-directory files (also named P
   and Q, by chance).  The D directory is empty, that is, contains no
   files or directories.  This structure may depicted graphically as...

            (unnamed root)
              /  |  \   \
             /   |   \   \
            A    X    B   Y
           /|\       / \
          / | \     /   \
         C  D  Z   P     Q
        / \
       /   \
      P     Q

   Given this structure, the following fully qualified pathnames exist.

         /
         /A
         /B
         /X
         /Y
         /A/C
         /A/D
         /A/Z
         /A/C/P
         /A/C/Q
         /B/P
         /B/Q




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   It is clear that none of the paths / /A /B or /A/D refer to the same
   directory, as the contents of each is different.  Nor do any of / /A
   /A/C or /A/D.  However /A/C and /B might be the same directory, there
   is insufficient information given to tell.  Any of the other
   pathnames (/X /Y /A/Z /A/C/P /A/C/Q /B/P and /B/Q) may refer to the
   same underlying files, in almost any combination.

   If the current working directory of the server-FTP is /A then the
   following pathnames, in addition to all the fully qualified
   pathnames, are valid

      C
      D
      Z
      C/P
      C/Q

   These all refer to the same files or directories as the corresponding
   fully qualified path with "/A/" prepended.

   That those pathnames all exist does not imply that the TVFS sever
   will necessarily grant any kind of access rights to the named paths,
   or that access to the same file via different pathnames will
   necessarily be granted equal rights.

   None of the following relative paths are valid when the current
   directory is /A

      A
      B
      X
      Y
      B/P
      B/Q
      P
      Q

   Any of those could be made valid by changing the server-FTP's current
   working directory to the appropriate directory.  Note that the paths
   "P" and "Q" might refer to different files depending upon which
   directory is selected to cause those to become valid TVFS relative
   paths.









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7.  Listings for Machine Processing (MLST and MLSD)

   The MLST and MLSD commands are intended to standardize the file and
   directory information returned by the server-FTP process.  These
   commands differ from the LIST command in that the format of the
   replies is strictly defined although extensible.

   Two commands are defined, MLST and MLSD.  MLST provides data about
   exactly the object named on its command line, and no others.  MLSD,
   on the other, lists the contents of a directory if a directory is
   named, otherwise a 501 reply is returned.  In either case, if no
   object is named, the current directory is assumed.  That will cause
   MLST to send a one-line response, describing the current directory
   itself, and MLSD to list the contents of the current directory.

   In the following, the term MLSx will be used wherever either MLST or
   MLSD may be inserted.

   The MLST and MLSD commands also extend the FTP protocol as presented
   in STD 9, RFC 959 [3] and STD 3, RFC 1123 [9] to allow that
   transmission of 8-bit data over the control connection.  Note this is
   not specifying character sets which are 8-bit, but specifying that
   FTP implementations are to specifically allow the transmission and
   reception of 8-bit bytes, with all bits significant, over the control
   connection.  That is, all 256 possible octet values are permitted.
   The MLSx command allows both UTF-8/Unicode and "raw" forms as
   arguments, and in responses both to the MLST and MLSD commands, and
   all other FTP commands which take pathnames as arguments.

7.1.  Format of MLSx Requests

   The MLST and MLSD commands each allow a single optional argument.
   This argument may be either a directory name or, for MLST only, a
   file name.  For these purposes, a "file name" is the name of any
   entity in the server NVFS which is not a directory.  Where TVFS is
   supported, any TVFS relative pathname valid in the current working
   directory, or any TVFS fully qualified pathname, may be given.  If a
   directory name is given then MLSD must return a listing of the
   contents of the named directory, otherwise it issues a 501 reply, and
   does not open a data connection.  In all cases for MLST, a single set
   of fact lines (usually a single fact line) containing the information
   about the named file or directory shall be returned over the control
   connection, without opening a data connection.

   If no argument is given then MLSD must return a listing of the
   contents of the current working directory, and MLST must return a
   listing giving information about the current working directory
   itself.  For these purposes, the contents of a directory are whatever



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   file or directory names (not pathnames) the server-PI will allow to
   be referenced when the current working directory is the directory
   named, and which the server-PI desires to reveal to the user-PI.
   Note that omitting the argument is the only defined way to obtain a
   listing of the current directory, unless a pathname that represents
   the directory happens to be known.  In particular, there is no
   defined shorthand name for the current directory.  This does not
   prohibit any particular server-PI implementing such a shorthand.

   No title, header, or summary, lines, or any other formatting, other
   than as is specified below, is ever returned in the output of an MLST
   or MLSD command.

   If the Client-FTP sends an invalid argument, the server-FTP MUST
   reply with an error code of 501.

   The syntax for the MLSx command is:

      mlst             = "MLst" [ SP pathname ] CRLF
      mlsd             = "MLsD" [ SP pathname ] CRLF

7.2.  Format of MLSx Response

   The format of a response to an MLSx command is as follows:

      mlst-response    = control-response / error-response
      mlsd-response    = ( initial-response final-response ) /
                         error-response

      control-response = "250-" [ response-message ] CRLF
                         1*( SP entry CRLF )
                         "250" [ SP response-message ] CRLF

      initial-response = "150" [ SP response-message ] CRLF
      final-response   = "226" SP response-message CRLF

      response-message = *TCHAR

      data-response    = *( entry CRLF )

      entry            = [ facts ] SP pathname
      facts            = 1*( fact ";" )
      fact             = factname "=" value
      factname         = "Size" / "Modify" / "Create" /
                         "Type" / "Unique" / "Perm" /
                         "Lang" / "Media-Type" / "CharSet" /
                         os-depend-fact / local-fact
      os-depend-fact   = <IANA assigned OS name> "." token



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      local-fact       = "X." token
      value            = *SCHAR

   Upon receipt of an MLSx command, the server will verify the
   parameter, and if invalid return an error-response.  For this
   purpose, the parameter should be considered to be invalid if the
   client issuing the command does not have permission to perform the
   requested operation.

   If the parameter is valid, then for an MLST command, the server-PI
   will send the first (leading) line of the control response, the entry
   for the pathname given, or the current directory if no pathname was
   provided, and the terminating line.  Normally exactly one entry would
   be returned, more entries are permitted only when required to
   represent a file that is to have multiple "Type" facts returned.  In
   this case, the pathname component of every response MUST be
   identical.

   Note that for MLST the fact set is preceded by a space.  That is
   provided to guarantee that the fact set cannot be accidentally
   interpreted as the terminating line of the control response, but is
   required even when that would not be possible.  Exactly one space
   exists between the set of facts and the pathname.  Where no facts are
   present, there will be exactly two leading spaces before the
   pathname.  No spaces are permitted in the facts, any other spaces in
   the response are to be treated as being a part of the pathname.

   If the command was an MLSD command, the server will open a data
   connection as indicated in section 3.2 of STD 9, RFC 959 [3].  If
   that fails, the server will return an error-response.  If all is OK,
   the server will return the initial-response, send the appropriate
   data-response over the new data connection, close that connection,
   and then send the final-response over the control connection.  The
   grammar above defines the format for the data-response, which defines
   the format of the data returned over the data connection established.

   The data connection opened for a MLSD response shall be a connection
   as if the "TYPE L 8", "MODE S", and "STRU F" commands had been given,
   whatever FTP transfer type, mode and structure had actually been set,
   and without causing those settings to be altered for future commands.
   That is, this transfer type shall be set for the duration of the data
   connection established for this command only.  While the content of
   the data sent can be viewed as a series of lines, implementations
   should note that there is no maximum line length defined.
   Implementations should be prepared to deal with arbitrarily long
   lines.





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   The facts part of the specification would contain a series of "file
   facts" about the file or directory named on the same line.  Typical
   information to be presented would include file size, last
   modification time, creation time, a unique identifier, and a
   file/directory flag.

   The complete format for a successful reply to the MLSD command would
   be:

      facts SP pathname CRLF
      facts SP pathname CRLF
      facts SP pathname CRLF
      ...

   Note that the format is intended for machine processing, not human
   viewing, and as such the format is very rigid.  Implementations MUST
   NOT vary the format by, for example, inserting extra spaces for
   readability, replacing spaces by tabs, including header or title
   lines, or inserting blank lines, or in any other way alter this
   format.  Exactly one space is always required after the set of facts
   (which may be empty).  More spaces may be present on a line if, and
   only if, the pathname presented contains significant spaces.  The set
   of facts must not contain any spaces anywhere inside it.  Facts
   should be provided in each output line only if they both provide
   relevant information about the file named on the same line, and they
   are in the set requested by the user-PI.  See section 7.9 (page 51).
   There is no requirement that the same set of facts be provided for
   each file, or that the facts presented occur in the same order for
   each file.

7.2.1.  Error Responses to MLSx commands

   Many of the 4xy and 5xy responses defined in section 4.2 of STD 9,
   RFC 959 [3] are possible in response to the MLST and MLSD commands.
   In particular, syntax errors can generate 500 or 501 replies.  Giving
   a pathname that exists but is not a directory as the argument to a
   MLSD command generates a 501 reply.  Giving a name that does not
   exist, or for which access permission (to obtain directory
   information as requested) is not granted will elicit a 550 reply.
   Other replies (530, 553, 503, 504, and any of the 4xy replies) are
   also possible in appropriate circumstances.

7.3.  File Name Encoding

   An FTP implementation supporting the MLSx commands must be 8-bit
   clean.  This is necessary in order to transmit UTF-8 encoded file
   names.  This specification recommends the use of UTF-8 encoded file




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   names.  FTP implementations SHOULD use UTF-8 whenever possible to
   encourage the maximum inter-operability.

   File names are not restricted to UTF-8, however treatment of
   arbitrary character encodings is not specified by this standard.
   Applications are encouraged to treat non-UTF-8 encodings of file
   names as octet sequences.

   Note that this encoding is unrelated to that of the contents of the
   file, even if the file contains character data.

   Further information about file name encoding for FTP may be found in
   "Internationalization of the File Transfer Protocol" [7].

7.3.1.  Notes about the File Name

   The file name returned in the MLST response should be the same name
   as was specified in the MLST command, or, where TVFS is supported, a
   fully qualified TVFS path naming the same file.  Where no argument
   was given to the MLST command, the server-PI may either include an
   empty file name in the response, or it may supply a name that refers
   to the current directory, if such a name is available.  Where TVFS is
   supported, a fully qualified pathname of the current directory SHOULD
   be returned.

   File names returned in the output from an MLSD command SHOULD be
   unqualified names within the directory named, or the current
   directory if no argument was given.  That is, the directory named in
   the MLSD command SHOULD NOT appear as a component of the file names
   returned.

   If the server-FTP process is able, and the "type" fact is being
   returned, it MAY return in the MLSD response, an entry whose type is
   "cdir", which names the directory from which the contents of the
   listing were obtained.  Where TVFS is supported, the name MAY be the
   fully qualified pathname of the directory, or MAY be any other
   pathname that is valid to refer to that directory from the current
   working directory of the server-FTP.  Where more than one name
   exists, multiple of these entries may be returned.  In a sense, the
   "cdir" entry can be viewed as a heading for the MLSD output.
   However, it is not required to be the first entry returned, and may
   occur anywhere within the listing.

   When TVFS is supported, a user-PI can refer to any file or directory
   in the listing by combining a type "cdir" name, with the appropriate
   name from the directory listing using the procedure defined in
   section 6.2.




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   Alternatively, whether TVFS is supported or not, the user-PI can
   issue a CWD command ([3]) giving a name of type "cdir" from the
   listing returned, and from that point reference the files returned in
   the MLSD response from which the cdir was obtained by using the file
   name components of the listing.

7.4.  Format of Facts

   The "facts" for a file in a reply to a MLSx command consist of
   information about that file.  The facts are a series of keyword=value
   pairs each followed by semi-colon (";") characters.  An individual
   fact may not contain a semi-colon in its name or value.  The complete
   series of facts may not contain the space character.  See the
   definition or "RCHAR" in section 2.1 for a list of the characters
   that can occur in a fact value.  Not all are applicable to all facts.

   A sample of a typical series of facts would be: (spread over two
   lines for presentation here only)

   size=4161;lang=en-US;modify=19970214165800;create=19961001124534;
   type=file;x.myfact=foo,bar;

7.5.  Standard Facts

   This document defines a standard set of facts as follows:

      size       -- Size in octets
      modify     -- Last modification time
      create     -- Creation time
      type       -- Entry type
      unique     -- Unique id of file/directory
      perm       -- File permissions, whether read, write, execute is
                    allowed for the login id.
      lang       -- Language of the file name per IANA [11] registry.
      media-type -- MIME media-type of file contents per IANA registry.
      charset    -- Character set per IANA registry (if not UTF-8)

   Fact names are case-insensitive.  Size, size, SIZE, and SiZe are the
   same fact.

   Further operating system specific keywords could be specified by
   using the IANA operating system name as a prefix (examples only):

      OS/2.ea   -- OS/2 extended attributes
      MACOS.rf  -- MacIntosh resource forks
      UNIX.mode -- Unix file modes (permissions)





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   Implementations may define keywords for experimental, or private use.
   All such keywords MUST begin with the two character sequence "x.".
   As type names are case independent, "x." and "X." are equivalent.
   For example:

      x.ver  -- Version information
      x.desc -- File description
      x.type -- File type

7.5.1.  The Type Fact

   The type fact needs a special description.  Part of the problem with
   current practices is deciding when a file is a directory.  If it is a
   directory, is it the current directory, a regular directory, or a
   parent directory?  The MLST specification makes this unambiguous
   using the type fact.  The type fact given specifies information about
   the object listed on the same line of the MLST response.

   Five values are possible for the type fact:

      file         -- a file entry
      cdir         -- the listed directory
      pdir         -- a parent directory
      dir          -- a directory or sub-directory
      OS.name=type -- an OS or file system dependent file type

   The syntax is defined to be:

      type-fact       = type-label "=" type-val
      type-label      = "Type"
      type-val        = "File" / "cdir" / "pdir" / "dir" /
                        os-type

   The value of the type fact (the "type-val") is a case independent
   string.

7.5.1.1.  type=file

   The presence of the type=file fact indicates the listed entry is a
   file containing non-system data.  That is, it may be transferred from
   one system to another of quite different characteristics, and perhaps
   still be meaningful.

7.5.1.2.  type=cdir

   The type=cdir fact indicates the listed entry contains a pathname of
   the directory whose contents are listed.  An entry of this type will
   only be returned as a part of the result of an MLSD command when the



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   type fact is included, and provides a name for the listed directory,
   and facts about that directory.  In a sense, it can be viewed as
   representing the title of the listing, in a machine friendly format.
   It may appear at any point of the listing, it is not restricted to
   appearing at the start, though frequently may do so, and may occur
   multiple times.  It MUST NOT be included if the type fact is not
   included, or there would be no way for the user-PI to distinguish the
   name of the directory from an entry in the directory.

   Where TVFS is supported by the server-FTP, this name may be used to
   construct pathnames with which to refer to the files and directories
   returned in the same MLSD output (see section 6.2).  These pathnames
   are only expected to work when the server-PI's position in the NVFS
   file tree is the same as its position when the MLSD command was
   issued, unless a fully qualified pathname results.

   Where TVFS is not supported, the only defined semantics associated
   with a "type=cdir" entry are that, provided the current working
   directory of the server-PI has not been changed, a pathname of type
   "cdir" may be used as an argument to a CWD command, which will cause
   the current directory of the server-PI to change so that the
   directory that was listed in its current working directory.

7.5.1.3.  type=dir

   If present, the type=dir entry gives the name of a directory.  Such
   an entry typically cannot be transferred from one system to another
   using RETR, etc., but should (permissions permitting) be able to be
   the object of an MLSD command.

7.5.1.4.  type=pdir

   If present, which will occur only in the response to a MLSD command
   when the type fact is included, the type=pdir entry represents a
   pathname of the parent directory of the listed directory.  As well as
   having the properties of a type=dir, a CWD command that uses the
   pathname from this entry should change the user to a parent directory
   of the listed directory.  If the listed directory is the current
   directory, a CDUP command may also have the effect of changing to the
   named directory.  User-FTP processes should note not all responses
   will include this information, and that some systems may provide
   multiple type=pdir responses.

   Where TVFS is supported, a "type=pdir" name may be a relative
   pathname, or a fully qualified pathname.  A relative pathname will be
   relative to the directory being listed, not to the current directory
   of the server-PI at the time.




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   For the purposes of this type value, a "parent directory" is any
   directory in which there is an entry of type=dir that refers to the
   directory in which the type=pdir entity was found.  Thus it is not
   required that all entities with type=pdir refer to the same
   directory.  The "unique" fact (if supported and supplied) can be used
   to determine whether there is a relationship between the type=pdir
   entries or not.

7.5.1.5.  System Defined Types

   Files types that are specific to a specific operating system, or file
   system, can be encoded using the "OS." type names.  The format is:

      os-type   = "OS." os-name "=" os-kind
      os-name   = <an IANA registered operating system name>
      os-kind   = token

   The "os-name" indicates the specific system type that supports the
   particular localtype.  OS specific types are registered by the IANA
   using the procedures specified in section 10.  The "os-kind" provides
   the system dependent information as to the type of the file listed.
   The os-name and os-kind strings in an os-type are case independent.
   "OS.unix=block" and "OS.Unix=BLOCK" represent the same type (or
   would, if such a type were registered.)

   Note: Where the underlying system supports a file type that is
   essentially an indirect pointer to another file, the NVFS
   representation of that type should normally be to represent the file
   that the reference indicates.  That is, the underlying basic file
   will appear more than once in the NVFS, each time with the "unique"
   fact (see immediately following section) containing the same value,
   indicating that the same file is represented by all such names.
   User-PIs transferring the file need then transfer it only once, and
   then insert their own form of indirect reference to construct
   alternate names where desired, or perhaps even copy the local file if
   that is the only way to provide two names with the same content.  A
   file which would be a reference to another file, if only the other
   file actually existed, may be represented in any OS dependent manner
   appropriate, or not represented at all.

7.5.1.6.  Multiple Types

   Where a file is such that it may validly, and sensibly, treated by
   the server-PI as being of more than one of the above types, then
   multiple entries should be returned, each with its own "Type" fact of
   the appropriate type, and each containing the same pathname.  This
   may occur, for example, with a structured file, which may contain
   sub-files, and where the server-PI permits the structured file to be



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   treated as a unit, or treated as a directory allowing the sub-files
   within it to be referenced.  When this is done, the pathname returned
   with each entry MUST be identical to the others representing the same
   file.

7.5.2.  The unique Fact

   The unique fact is used to present a unique identifier for a file or
   directory in the NVFS accessed via a server-FTP process.  The value
   of this fact should be the same for any number of pathnames that
   refer to the same underlying file.  The fact should have different
   values for names that reference distinct files.  The mapping between
   files, and unique fact tokens should be maintained, and remain
   consistent, for at least the lifetime of the control connection from
   user-PI to server-PI.

      unique-fact  = "Unique" "=" token

   This fact would be expected to be used by server-FTPs whose host
   system allows things such as symbolic links so that the same file may
   be represented in more than one directory on the server.  The only
   conclusion that should be drawn is that if two different names each
   have the same value for the unique fact, they refer to the same
   underlying object.  The value of the unique fact (the token) should
   be considered an opaque string for comparison purposes, and is a case
   dependent value.  The tokens "A" and "a" do not represent the same
   underlying object.

7.5.3.  The modify Fact

   The modify fact is used to determine the last time the content of the
   file (or directory) indicated was modified.  Any change of substance
   to the file should cause this value to alter.  That is, if a change
   is made to a file such that the results of a RETR command would
   differ, then the value of the modify fact should alter.  User-PIs
   should not assume that a different modify fact value indicates that
   the file contents are necessarily different than when last retrieved.
   Some systems may alter the value of the modify fact for other
   reasons, though this is discouraged wherever possible.  Also a file
   may alter, and then be returned to its previous content, which would
   often be indicated as two incremental alterations to the value of the
   modify fact.

   For directories, this value should alter whenever a change occurs to
   the directory such that different file names would (or might) be
   included in MLSD output of that directory.

      modify-fact  = "Modify" "=" time-val



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7.5.4.  The create Fact

   The create fact indicates when a file, or directory, was first
   created.  Exactly what "creation" is for this purpose is not
   specified here, and may vary from server to server.  About all that
   can be said about the value returned is that it can never indicate a
   later time than the modify fact.

      create-fact  = "Create" "=" time-val

   Implementation Note: Implementors of this fact on UNIX(TM) systems
      should note that the unix "stat" "st_ctime" field does not give
      creation time, and that unix file systems do not record creation
      time at all.  Unix (and POSIX) implementations will normally not
      include this fact.

7.5.5.  The perm Fact

   The perm fact is used to indicate access rights the current FTP user
   has over the object listed.  Its value is always an unordered
   sequence of alphabetic characters.

      perm-fact    = "Perm" "=" *pvals
      pvals        = "a" / "c" / "d" / "e" / "f" /
                     "l" / "m" / "p" / "r" / "w"

   There are ten permission indicators currently defined.  Many are
   meaningful only when used with a particular type of object.  The
   indicators are case independent, "d" and "D" are the same indicator.

   The "a" permission applies to objects of type=file, and indicates
   that the APPE (append) command may be applied to the file named.

   The "c" permission applies to objects of type=dir (and type=pdir,
   type=cdir).  It indicates that files may be created in the directory
   named.  That is, that a STOU command is likely to succeed, and that
   STOR and APPE commands might succeed if the file named did not
   previously exist, but is to be created in the directory object that
   has the "c" permission.  It also indicates that the RNTO command is
   likely to succeed for names in the directory.

   The "d" permission applies to all types.  It indicates that the
   object named may be deleted, that is, that the RMD command may be
   applied to it if it is a directory, and otherwise that the DELE
   command may be applied to it.

   The "e" permission applies to the directory types.  When set on an
   object of type=dir, type=cdir, or type=pdir it indicates that a CWD



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   command naming the object should succeed, and the user should be able
   to enter the directory named.  For type=pdir it also indicates that
   the CDUP command may succeed (if this particular pathname is the one
   to which a CDUP would apply.)

   The "f" permission for objects indicates that the object named may be
   renamed - that is, may be the object of an RNFR command.

   The "l" permission applies to the directory file types, and indicates
   that the listing commands, LIST, NLST, and MLSD may be applied to the
   directory in question.

   The "m" permission applies to directory types, and indicates that the
   MKD command may be used to create a new directory within the
   directory under consideration.

   The "p" permission applies to directory types, and indicates that
   objects in the directory may be deleted, or (stretching naming a
   little) that the directory may be purged.  Note: it does not indicate
   that the RMD command may be used to remove the directory named
   itself, the "d" permission indicator indicates that.

   The "r" permission applies to type=file objects, and for some
   systems, perhaps to other types of objects, and indicates that the
   RETR command may be applied to that object.

   The "w" permission applies to type=file objects, and for some
   systems, perhaps to other types of objects, and indicates that the
   STOR command may be applied to the object named.

   Note: That a permission indicator is set can never imply that the
      appropriate command is guaranteed to work -- just that it might.
      Other system specific limitations, such as limitations on
      available space for storing files, may cause an operation to fail,
      where the permission flags may have indicated that it was likely
      to succeed.  The permissions are a guide only.

   Implementation note: The permissions are described here as they apply
      to FTP commands.  They may not map easily into particular
      permissions available on the server's operating system.  Servers
      are expected to synthesize these permission bits from the
      permission information available from operating system.  For
      example, to correctly determine whether the "D" permission bit
      should be set on a directory for a server running on the UNIX(TM)
      operating system, the server should check that the directory named
      is empty, and that the user has write permission on both the
      directory under consideration, and its parent directory.




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      Some systems may have more specific permissions than those listed
      here, such systems should map those to the flags defined as best
      they are able.  Other systems may have only more broad access
      controls.  They will generally have just a few possible
      permutations of permission flags, however they should attempt to
      correctly represent what is permitted.

7.5.6.  The lang Fact

   The lang fact describes the natural language of the file name for use
   in display purposes.  Values used here should be taken from the
   language registry of the IANA.  See [12] for the syntax, and
   procedures, related to language tags.

      lang-fact  = "Lang" "=" token

   Server-FTP implementations MUST NOT guess language values.  Language
   values must be determined in an unambiguous way such as file system
   tagging of language or by user configuration.  Note that the lang
   fact provides no information at all about the content of a file, only
   about the encoding of its name.

7.5.7.  The size Fact

   The size fact applies to non-directory file types and should always
   reflect the approximate size of the file.  This should be as accurate
   as the server can make it, without going to extraordinary lengths,
   such as reading the entire file.  The size is expressed in units of
   octets of data in the file.

   Given limitations in some systems, Client-FTP implementations must
   understand this size may not be precise and may change between the
   time of a MLST and RETR operation.

   Clients that need highly accurate size information for some
   particular reason should use the SIZE command as defined in section
   4.  The most common need for this accuracy is likely to be in
   conjunction with the REST command described in section 5.  The size
   fact, on the other hand, should be used for purposes such as
   indicating to a human user the approximate size of the file to be
   transferred, and perhaps to give an idea of expected transfer
   completion time.

      size-fact  = "Size" "=" 1*DIGIT







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7.5.8.  The media-type Fact

   The media-type fact represents the IANA media type of the file named,
   and applies only to non-directory types.  The list of values used
   must follow the guidelines set by the IANA registry.

      media-type  = "Media-Type" "=" <per IANA guidelines>

   Server-FTP implementations MUST NOT guess media type values.  Media
   type values must be determined in an unambiguous way such as file
   system tagging of media-type or by user configuration.  This fact
   gives information about the content of the file named.  Both the
   primary media type, and any appropriate subtype should be given,
   separated by a slash "/" as is traditional.

7.5.9.  The charset Fact

   The charset fact provides the IANA character set name, or alias, for
   the encoded pathnames in a MLSx response.  The default character set
   is UTF-8 unless specified otherwise.  FTP implementations SHOULD use
   UTF-8 if possible to encourage maximum inter-operability.  The value
   of this fact applies to the pathname only, and provides no
   information about the contents of the file.

      charset-type  = "Charset" "=" token

7.5.10.  Required Facts

   Servers are not required to support any particular set of the
   available facts.  However, servers SHOULD, if conceivably possible,
   support at least the type, perm, size, unique, and modify facts.

7.6.  System Dependent and Local Facts

   By using an system dependent fact, or a local fact, a server-PI may
   communicate to the user-PI information about the file named that is
   peculiar to the underlying file system.

7.6.1.  System Dependent Facts

   System dependent fact names are labeled by prefixing a label
   identifying the specific information returned by the name of the
   appropriate operating system from the IANA maintained list of
   operating system names.

   The value of an OS dependent fact may be whatever is appropriate to
   convey the information available.  It must be encoded as a "token" as
   defined in section 2.1 however.



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   In order to allow reliable inter-operation between users of system
   dependent facts, the IANA will maintain a registry of system
   dependent fact names, their syntax, and the interpretation to be
   given to their values.  Registrations of system dependent facts are
   to be accomplished according to the procedures of section 10.

7.6.2.  Local Facts

   Implementations may also make available other facts of their own
   choosing.  As the method of interpretation of such information will
   generally not be widely understood, server-PIs should be aware that
   clients will typically ignore any local facts provided.  As there is
   no registration of locally defined facts, it is entirely possible
   that different servers will use the same local fact name to provide
   vastly different information.  Hence user-PIs should be hesitant
   about making any use of any information in a locally defined fact
   without some other specific assurance that the particular fact is one
   that they do comprehend.

   Local fact names all begin with the sequence "X.".  The rest of the
   name is a "token" (see section 2.1).  The value of a local fact can
   be anything at all, provided it can be encoded as a "token".

7.7.  MLSx Examples

   The following examples are all taken from dialogues between existing
   FTP clients and servers.  Because of this, not all possible
   variations of possible response formats are shown in the examples.
   This should not be taken as limiting the options of other server
   implementors.  Where the examples show OS dependent information, that
   is to be treated as being purely for the purposes of demonstration of
   some possible OS specific information that could be defined.  As at
   the time of the writing of this document, no OS specific facts or
   file types have been defined, the examples shown here should not be
   treated as in any way to be preferred over other possible similar
   definitions.  Consult the IANA registries to determine what types and
   facts have been defined.  Finally also beware that as the examples
   shown are taken from existing implementations, coded before this
   document was completed, the possibility of variations between the
   text of this document and the examples exists.  In any such case of
   inconsistency, the example is to be treated as incorrect.

   In the examples shown, only relevant commands and responses have been
   included.  This is not to imply that other commands (including
   authentication, directory modification, PORT or PASV commands, or
   similar) would not be present in an actual connection, or were not,
   in fact, actually used in the examples before editing.  Note also
   that the formats shown are those that are transmitted between client



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   and server, not formats that would normally ever be reported to the
   user of the client.

7.7.1.  Simple MLST

C> PWD
S> 257 "/tmp" is current directory.
C> MLst cap60.pl198.tar.gz
S> 250- Listing cap60.pl198.tar.gz
S>  Type=file;Size=1024990;Perm=r; /tmp/cap60.pl198.tar.gz
S> 250 End

   The client first asked to be told the current directory of the
   server.  This was purely for the purposes of clarity of this example.
   The client then requested facts about a specific file.  The server
   returned the "250-" first control-response line, followed by a single
   line of facts about the file, followed by the terminating "250 "
   line.  The text on the control-response line and the terminating line
   can be anything the server decides to send.  Notice that the fact
   line is indented by a single space.  Notice also that there are no
   spaces in the set of facts returned, until the single space before
   the file name.  The file name returned on the fact line is a fully
   qualified pathname of the file listed.  The facts returned show that
   the line refers to a file, that file contains approximately 1024990
   bytes, though more or less than that may be transferred if the file
   is retrieved, and a different number may be required to store the
   file at the client's file store, and the connected user has
   permission to retrieve the file but not to do anything else
   particularly interesting.

7.7.2.  MLST of a directory

C> PWD
S> 257 "/" is current directory.
C> MLst tmp
S> 250- Listing tmp
S>  Type=dir;Modify=19981107085215;Perm=el; /tmp
S> 250 End

   Again the PWD is just for the purposes of demonstration for the
   example.  The MLST fact line this time shows that the file listed is
   a directory, that it was last modified at 08:52:15 on the 7th of
   November, 1998 UTC, and that the user has permission to enter the
   directory, and to list its contents, but not to modify it in any way.
   Again, the fully qualified pathname of the directory listed is given.






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7.7.3.  MLSD of a directory

C> MLSD tmp
S> 150 BINARY connection open for MLSD tmp
D> Type=cdir;Modify=19981107085215;Perm=el; tmp
D> Type=cdir;Modify=19981107085215;Perm=el; /tmp
D> Type=pdir;Modify=19990112030508;Perm=el; ..
D> Type=file;Size=25730;Modify=19940728095854;Perm=; capmux.tar.z
D> Type=file;Size=1830;Modify=19940916055648;Perm=r; hatch.c
D> Type=file;Size=25624;Modify=19951003165342;Perm=r; MacIP-02.txt
D> Type=file;Size=2154;Modify=19950501105033;Perm=r; uar.netbsd.patch
D> Type=file;Size=54757;Modify=19951105101754;Perm=r; iptnnladev.1.0.sit.hqx
D> Type=file;Size=226546;Modify=19970515023901;Perm=r; melbcs.tif
D> Type=file;Size=12927;Modify=19961025135602;Perm=r; tardis.1.6.sit.hqx
D> Type=file;Size=17867;Modify=19961025135602;Perm=r; timelord.1.4.sit.hqx
D> Type=file;Size=224907;Modify=19980615100045;Perm=r; uar.1.2.3.sit.hqx
D> Type=file;Size=1024990;Modify=19980130010322;Perm=r; cap60.pl198.tar.gz
S> 226 MLSD completed

   In this example notice that there is no leading space on the fact
   lines returned over the data connection.  Also notice that two lines
   of "type=cdir" have been given.  These show two alternate names for
   the directory listed, one a fully qualified pathname, and the other a
   local name relative to the servers current directory when the MLSD
   was performed.  Note that all other file names in the output are
   relative to the directory listed, though the server could, if it
   chose, give a fully qualified pathname for the "type=pdir" line.
   This server has chosen not to.  The other files listed present a
   fairly boring set of files that are present in the listed directory.
   Note that there is no particular order in which they are listed.
   They are not sorted by file name, by size, or by modify time.  Note
   also that the "perm" fact has an empty value for the file
   "capmux.tar.z" indicating that the connected user has no permissions
   at all for that file.  This server has chosen to present the "cdir"
   and "pdir" lines before the lines showing the content of the
   directory, it is not required to do so.  The "size" fact does not
   provide any meaningful information for a directory, so is not
   included in the fact lines for the directory types shown.













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7.7.4.  A More Complex Example

C> MLst test
S> 250- Listing test
S>  Type=dir;Perm=el;Unique=keVO1+ZF4 test
S> 250 End
C> MLSD test
S> 150 BINARY connection open for MLSD test
D> Type=cdir;Perm=el;Unique=keVO1+ZF4; test
D> Type=pdir;Perm=e;Unique=keVO1+d?3; ..
D> Type=OS.unix=slink:/foobar;Perm=;Unique=keVO1+4G4; foobar
D> Type=OS.unix=chr-13/29;Perm=;Unique=keVO1+5G4; device
D> Type=OS.unix=blk-11/108;Perm=;Unique=keVO1+6G4; block
D> Type=file;Perm=awr;Unique=keVO1+8G4; writable
D> Type=dir;Perm=cpmel;Unique=keVO1+7G4; promiscuous
D> Type=dir;Perm=;Unique=keVO1+1t2; no-exec
D> Type=file;Perm=r;Unique=keVO1+EG4; two words
D> Type=file;Perm=r;Unique=keVO1+IH4;  leading space
D> Type=file;Perm=r;Unique=keVO1+1G4; file1
D> Type=dir;Perm=cpmel;Unique=keVO1+7G4; incoming
D> Type=file;Perm=r;Unique=keVO1+1G4; file2
D> Type=file;Perm=r;Unique=keVO1+1G4; file3
D> Type=file;Perm=r;Unique=keVO1+1G4; file4
S> 226 MLSD completed
C> MLSD test/incoming
S> 150 BINARY connection open for MLSD test/incoming
D> Type=cdir;Perm=cpmel;Unique=keVO1+7G4; test/incoming
D> Type=pdir;Perm=el;Unique=keVO1+ZF4; ..
D> Type=file;Perm=awdrf;Unique=keVO1+EH4; bar
D> Type=file;Perm=awdrf;Unique=keVO1+LH4;
D> Type=file;Perm=rf;Unique=keVO1+1G4; file5
D> Type=file;Perm=rf;Unique=keVO1+1G4; file6
D> Type=dir;Perm=cpmdelf;Unique=keVO1+!s2; empty
S> 226 MLSD completed

   For the purposes of this example the fact set requested has been
   modified to delete the "size" and "modify" facts, and add the
   "unique" fact.  First, facts about a file name have been obtained via
   MLST.  Note that no fully qualified pathname was given this time.
   That was because the server was unable to determine that information.
   Then having determined that the file name represents a directory,
   that directory has been listed.  That listing also shows no fully
   qualified pathname, for the same reason, thus has but a single
   "type=cdir" line.  This directory (which was created especially for
   the purpose) contains several interesting files.  There are some with
   OS dependent file types, several sub-directories, and several
   ordinary files.




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   Not much can be said here about the OS dependent file types, as none
   of the information shown there should be treated as any more than
   possibilities.  It can be seen that the OS type of the server is
   "unix" though, which is one of the OS types in the IANA registry of
   Operating System names.

   Of the three directories listed, "no-exec" has no permission granted
   to this user to access at all.  From the "Unique" fact values, it can
   be determined that "promiscuous" and "incoming" in fact represent the
   same directory.  Its permissions show that the connected user has
   permission to do essentially anything other than to delete the
   directory.  That directory was later listed.  It happens that the
   directory can not be deleted because it is not empty.

   Of the normal files listed, two contain spaces in their names.  The
   file called " leading space" actually contains two spaces in its
   name, one before the "l" and one between the "g" and the "s".  The
   two spaces that separate the facts from the visible part of the
   pathname make that clear.  The file "writable" has the "a" and "w"
   permission bits set, and consequently the connected user should be
   able to STOR or APPE to that file.

   The other four file names, "file1", "file2", "file3", and "file4" all
   represent the same underlying file, as can be seen from the values of
   the "unique" facts of each.  It happens that "file1" and "file2" are
   Unix "hard" links, and that "file3" and "file4" are "soft" or
   "symbolic" links to the first two.  None of that information is
   available via standard MLST facts, it is sufficient for the purposes
   of FTP to note that all represent the same file, and that the same
   data would be fetched no matter which of them was retrieved, and that
   all would be simultaneously modified were data stored in any.

   Finally, the sub-directory "incoming" is listed.  Since "promiscuous"
   is the same directory there would be no point listing it as well.  In
   that directory, the files "file5" and "file6" represent still more
   names for the "file1" file we have seen before.  Notice the entry
   between that for "bar" and "file5".  Though it is not possible to
   easily represent it in this document, that shows a file with a name
   comprising exactly three spaces ("   ").  A client will have no
   difficulty determining that name from the output presented to it
   however.  The directory "empty" is, as its name implies, empty,
   though that is not shown here.  It can, however, be deleted, as can
   file "bar" and the file whose name is three spaces.  All the files
   that reside in this directory can be renamed.  This is a consequence
   of the UNIX semantics of the directory that contains them being
   modifiable.





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7.7.5.  More Accurate Time Information

C> MLst file1
S> 250- Listing file1
S>  Type=file;Modify=19990929003355.237; file1
S> 250 End

   In this example, the server-FTP is indicating that "file1" was last
   modified 237 milliseconds after 00:33:55 UTC on the 29th of
   September, 1999.

7.7.6.  A Different Server

C> MLST
S> 250-Begin
S>  type=dir;unique=AQkAAAAAAAABCAAA; /
S> 250 End.
C> MLSD
S> 150 Opening ASCII mode data connection for MLS.
D> type=cdir;unique=AQkAAAAAAAABCAAA; /
D> type=dir;unique=AQkAAAAAAAABEAAA; bin
D> type=dir;unique=AQkAAAAAAAABGAAA; etc
D> type=dir;unique=AQkAAAAAAAAB8AwA; halflife
D> type=dir;unique=AQkAAAAAAAABoAAA; incoming
D> type=dir;unique=AQkAAAAAAAABIAAA; lib
D> type=dir;unique=AQkAAAAAAAABWAEA; linux
D> type=dir;unique=AQkAAAAAAAABKAEA; ncftpd
D> type=dir;unique=AQkAAAAAAAABGAEA; outbox
D> type=dir;unique=AQkAAAAAAAABuAAA; quake2
D> type=dir;unique=AQkAAAAAAAABQAEA; winstuff
S> 226 Listing completed.
C> MLSD linux
S> 150 Opening ASCII mode data connection for MLS.
D> type=cdir;unique=AQkAAAAAAAABWAEA; /linux
D> type=pdir;unique=AQkAAAAAAAABCAAA; /
D> type=dir;unique=AQkAAAAAAAABeAEA; firewall
D> type=file;size=12;unique=AQkAAAAAAAACWAEA; helo_world
D> type=dir;unique=AQkAAAAAAAABYAEA; kernel
D> type=dir;unique=AQkAAAAAAAABmAEA; scripts
D> type=dir;unique=AQkAAAAAAAABkAEA; security
S> 226 Listing completed.
C> MLSD linux/kernel
S> 150 Opening ASCII mode data connection for MLS.
D> type=cdir;unique=AQkAAAAAAAABYAEA; /linux/kernel
D> type=pdir;unique=AQkAAAAAAAABWAEA; /linux
D> type=file;size=6704;unique=AQkAAAAAAAADYAEA; k.config
D> type=file;size=7269221;unique=AQkAAAAAAAACYAEA; linux-2.0.36.tar.gz
D> type=file;size=12514594;unique=AQkAAAAAAAAEYAEA; linux-2.1.130.tar.gz



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S> 226 Listing completed.

   Note that this server returns its "unique" fact value in quite a
   different format.  It also returns fully qualified pathnames for the
   "pdir" entry.

7.7.7.  Some IANA Files

C> MLSD
S> 150 BINARY connection open for MLSD .
D> Type=cdir;Modify=19990219183438; /iana/assignments
D> Type=pdir;Modify=19990112030453; ..
D> Type=dir;Modify=19990219073522; media-types
D> Type=dir;Modify=19990112033515; character-set-info
D> Type=dir;Modify=19990112033529; languages
D> Type=file;Size=44242;Modify=19990217230400; character-sets
D> Type=file;Size=1947;Modify=19990209215600; operating-system-names
S> 226 MLSD completed
C> MLSD media-types
S> 150 BINARY connection open for MLSD media-types
D> Type=cdir;Modify=19990219073522; media-types
D> Type=cdir;Modify=19990219073522; /iana/assignments/media-types
D> Type=pdir;Modify=19990219183438; ..
D> Type=dir;Modify=19990112033045; text
D> Type=dir;Modify=19990219183442; image
D> Type=dir;Modify=19990112033216; multipart
D> Type=dir;Modify=19990112033254; video
D> Type=file;Size=30249;Modify=19990218032700; media-types
S> 226 MLSD completed
C> MLSD character-set-info
S> 150 BINARY connection open for MLSD character-set-info
D> Type=cdir;Modify=19990112033515; character-set-info
D> Type=cdir;Modify=19990112033515; /iana/assignments/character-set-info
D> Type=pdir;Modify=19990219183438; ..
D> Type=file;Size=1234;Modify=19980903020400; windows-1251
D> Type=file;Size=4557;Modify=19980922001400; tis-620
D> Type=file;Size=801;Modify=19970324130000; ibm775
D> Type=file;Size=552;Modify=19970320130000; ibm866
D> Type=file;Size=922;Modify=19960505140000; windows-1258
S> 226 MLSD completed
C> MLSD languages
S> 150 BINARY connection open for MLSD languages
D> Type=cdir;Modify=19990112033529; languages
D> Type=cdir;Modify=19990112033529; /iana/assignments/languages
D> Type=pdir;Modify=19990219183438; ..
D> Type=file;Size=2391;Modify=19980309130000; default
D> Type=file;Size=943;Modify=19980309130000; tags
D> Type=file;Size=870;Modify=19971026130000; navajo



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D> Type=file;Size=699;Modify=19950911140000; no-bok
S> 226 MLSD completed
C> PWD
S> 257 "/iana/assignments" is current directory.

   This example shows some of the IANA maintained files that are
   relevant for this specification in MLSD format.  Note that these
   listings have been edited by deleting many entries, the actual
   listings are much longer.

7.7.8.  A Stress Test of Case (In)dependence

   The following example is intended to make clear some cases where case
   dependent strings are permitted in the MLSx commands, and where case
   independent strings are required.

   Note first that the "MLSD" command, shown here as "MlsD" is case
   independent.  Clients may issue this command in any case, or
   combination of cases, they desire.  This is the case for all FTP
   commands.

C> MlsD
S> 150 BINARY connection open for MLSD .
D> Type=pdir;Modify=19990929011228;Perm=el;Unique=keVO1+ZF4; ..
D> Type=file;Size=4096;Modify=19990929011440;Perm=r;Unique=keVO1+Bd8; FILE2
D> Type=file;Size=4096;Modify=19990929011440;Perm=r;Unique=keVO1+aG8; file3
D> Type=file;Size=4096;Modify=19990929011440;Perm=r;Unique=keVO1+ag8; FILE3
D> Type=file;Size=4096;Modify=19990929011440;Perm=r;Unique=keVO1+bD8; file1
D> Type=file;Size=4096;Modify=19990929011440;Perm=r;Unique=keVO1+bD8; file2
D> Type=file;Size=4096;Modify=19990929011440;Perm=r;Unique=keVO1+Ag8; File3
D> Type=file;Size=4096;Modify=19990929011440;Perm=r;Unique=keVO1+bD8; File1
D> Type=file;Size=4096;Modify=19990929011440;Perm=r;Unique=keVO1+Bd8; File2
D> Type=file;Size=4096;Modify=19990929011440;Perm=r;Unique=keVO1+bd8; FILE1
S> 226 MLSD completed

   Next, notice the labels of the facts.  These are also case-
   independent strings; the server-FTP is permitted to return them in
   any case desired.  User-FTP must be prepared to deal with any case,
   though it may do this by mapping the labels to a common case if
   desired.

   Then, notice that there are nine objects of "type" file returned.  In
   a case-independent NVFS these would represent three different file
   names, "file1", "file2", and "file3".  With a case-dependent NVFS all
   nine represent different file names.  Either is possible, server-FTPs
   may implement a case dependent or a case independent NVFS.  User-FTPs
   must allow for case dependent selection of files to manipulate on the
   server.



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   Lastly, notice that the value of the "unique" fact is case dependent.
   In the example shown, "file1", "File1", and "file2" all have the same
   "unique" fact value "keVO1+bD8", and thus all represent the same
   underlying file.  On the other hand, "FILE1" has a different "unique"
   fact value ("keVO1+bd8") and hence represents a different file.
   Similarly, "FILE2" and "File2" are two names for the same underlying
   file, whereas "file3", "File3" and "FILE3" all represent different
   underlying files.

   That the approximate sizes ("size" fact) and last modification times
   ("modify" fact) are the same in all cases might be no more than a
   coincidence.

   It is not suggested that the operators of server-FTPs create an NVFS
   that stresses the protocols to this extent; however, both user and
   server implementations must be prepared to deal with such extreme
   examples.

7.7.9.  Example from Another Server

C> MlsD
S> 150 File Listing Follows in IMAGE / Binary mode.
D> type=cdir;modify=19990426150227;perm=el; /MISC
D> type=pdir;modify=19791231130000;perm=el; /
D> type=dir;modify=19990426150227;perm=el; CVS
D> type=dir;modify=19990426150228;perm=el; SRC
S> 226 Transfer finished successfully.
C> MlsD src
S> 150 File Listing Follows in IMAGE / Binary mode.
D> type=cdir;modify=19990426150228;perm=el; /MISC/src
D> type=pdir;modify=19990426150227;perm=el; /MISC
D> type=dir;modify=19990426150228;perm=el; CVS
D> type=dir;modify=19990426150228;perm=el; INSTALL
D> type=dir;modify=19990426150230;perm=el; INSTALLI
D> type=dir;modify=19990426150230;perm=el; TREES
S> 226 Transfer finished successfully.
C> MlsD src/install
S> 150 File Listing Follows in IMAGE / Binary mode.
D> type=cdir;modify=19990426150228;perm=el; /MISC/src/install
D> type=pdir;modify=19990426150228;perm=el; /MISC/src
D> type=file;modify=19990406234304;perm=r;size=20059; BOOTPC.C
D> type=file;modify=19980401170153;perm=r;size=278; BOOTPC.H
D> type=file;modify=19990413153736;perm=r;size=54220; BOOTPC.O
D> type=file;modify=19990223044003;perm=r;size=3389; CDROM.C
D> type=file;modify=19990413153739;perm=r;size=30192; CDROM.O
D> type=file;modify=19981119155324;perm=r;size=1055; CHANGELO
D> type=file;modify=19981204171040;perm=r;size=8297; COMMANDS.C
D> type=file;modify=19980508041749;perm=r;size=580; COMMANDS.H



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                                                 ...
D> type=file;modify=19990419052351;perm=r;size=54264; URLMETHO.O
D> type=file;modify=19980218161629;perm=r;size=993; WINDOWS.C
D> type=file;modify=19970912154859;perm=r;size=146; WINDOWS.H
D> type=file;modify=19990413153731;perm=r;size=16812; WINDOWS.O
D> type=file;modify=19990322174959;perm=r;size=129; _CVSIGNO
D> type=file;modify=19990413153640;perm=r;size=82536; _DEPEND
S> 226 Transfer finished successfully.
C> MLst src/install/windows.c
S> 250-Listing src/install/windows.c
S>  type=file;perm=r;size=993; /misc/src/install/windows.c
S> 250 End
S> ftp> mlst SRC/INSTALL/WINDOWS.C
C> MLst SRC/INSTALL/WINDOWS.C
S> 250-Listing SRC/INSTALL/WINDOWS.C
S>  type=file;perm=r;size=993; /misc/SRC/INSTALL/WINDOWS.C
S> 250 End

   Note that this server gives fully qualified pathnames for the "pdir"
   and "cdir" entries in MLSD listings.  Also notice that this server
   does, though it is not required to, sort its directory listing
   outputs.  That may be an artifact of the underlying file system
   access mechanisms of course.  Finally notice that the NVFS supported
   by this server, in contrast to the earlier ones, implements its
   pathnames in a case independent manner.  The server seems to return
   files using the case in which they were requested, when the name was
   sent by the client, and otherwise uses an algorithm known only to
   itself to select the case of the names it returns.

7.7.10.  A Server Listing Itself

C> MLst f
S> 250-MLST f
S>  Type=dir;Modify=20000710052229;Unique=AAD/AAAABIA; f
S> 250 End
C> CWD f
S> 250 CWD command successful.
C> MLSD
S> 150 Opening ASCII mode data connection for 'MLSD'.
D> Type=cdir;Unique=AAD/AAAABIA; .
D> Type=pdir;Unique=AAD/AAAAAAI; ..
D> Type=file;Size=987;Unique=AAD/AAAABIE; Makefile
D> Type=file;Size=20148;Unique=AAD/AAAABII; conf.c
D> Type=file;Size=11111;Unique=AAD/AAAABIM; extern.h
D> Type=file;Size=38721;Unique=AAD/AAAABIQ; ftpcmd.y
D> Type=file;Size=17922;Unique=AAD/AAAABIU; ftpd.8
D> Type=file;Size=60732;Unique=AAD/AAAABIY; ftpd.c
D> Type=file;Size=3127;Unique=AAD/AAAABIc; logwtmp.c



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D> Type=file;Size=2294;Unique=AAD/AAAABIg; pathnames.h
D> Type=file;Size=7605;Unique=AAD/AAAABIk; popen.c
D> Type=file;Size=9951;Unique=AAD/AAAABIo; ftpd.conf.5
D> Type=file;Size=5023;Unique=AAD/AAAABIs; ftpusers.5
D> Type=file;Size=3547;Unique=AAD/AAAABIw; logutmp.c
D> Type=file;Size=2064;Unique=AAD/AAAABI0; version.h
D> Type=file;Size=20420;Unique=AAD/AAAAAAM; cmds.c
D> Type=file;Size=15864;Unique=AAD/AAAAAAg; ls.c
D> Type=file;Size=2898;Unique=AAD/AAAAAAk; ls.h
D> Type=file;Size=2769;Unique=AAD/AAAAAAo; lsextern.h
D> Type=file;Size=2042;Unique=AAD/AAAAAAs; stat_flags.h
D> Type=file;Size=5708;Unique=AAD/AAAAAAw; cmp.c
D> Type=file;Size=9280;Unique=AAD/AAAAAA0; print.c
D> Type=file;Size=4657;Unique=AAD/AAAAAA4; stat_flags.c
D> Type=file;Size=2664;Unique=AAD/AAAAAA8; util.c
D> Type=file;Size=10383;Unique=AAD/AAAABJ0; ftpd.conf.cat5
D> Type=file;Size=3631;Unique=AAD/AAAABJ4; ftpusers.cat5
D> Type=file;Size=17729;Unique=AAD/AAAABJ8; ftpd.cat8
S> 226 MLSD complete.

   This examples shows yet another server implementation, showing a
   listing of its own source code.  Note that this implementation does
   not include the fully qualified path name in its "cdir" and "pdir"
   entries, nor in the output from "MLST".  Also note that the facts
   requested were modified between the "MLST" and "MLSD" commands,
   though that exchange has not been shown here.

7.7.11.  A Server with a Difference

C> PASV
S> 227 Entering Passive Mode (127,0,0,1,255,46)
C> MLSD
S> 150 I tink I tee a trisector tree
D> Type=file;Unique=aaaaafUYqaaa;Perm=rf;Size=15741; x
D> Type=cdir;Unique=aaaaacUYqaaa;Perm=cpmel; /
D> Type=file;Unique=aaaaajUYqaaa;Perm=rf;Size=5760; x4
D> Type=dir;Unique=aaabcaUYqaaa;Perm=elf; sub
D> Type=file;Unique=aaaaagUYqaaa;Perm=rf;Size=8043; x1
D> Type=dir;Unique=aaab8aUYqaaa;Perm=cpmelf; files
D> Type=file;Unique=aaaaahUYqaaa;Perm=rf;Size=4983; x2
D> Type=file;Unique=aaaaaiUYqaaa;Perm=rf;Size=6854; x3
S> 226 That's all folks...
C> CWD sub
S> 250 CWD command successful.
C> PWD
S> 257 "/sub" is current directory.
C> PASV
S> 227 Entering Passive Mode (127,0,0,1,255,44)



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C> MLSD
S> 150 I tink I tee a trisector tree
D> Type=dir;Unique=aaabceUYqaaa;Perm=elf; dir
D> Type=file;Unique=aaabcbUYqaaa;Perm=rf;Size=0; y1
D> Type=file;Unique=aaabccUYqaaa;Perm=rf;Size=0; y2
D> Type=file;Unique=aaabcdUYqaaa;Perm=rf;Size=0; y3
D> Type=pdir;Unique=aaaaacUYqaaa;Perm=cpmel; /
D> Type=pdir;Unique=aaaaacUYqaaa;Perm=cpmel; ..
D> Type=cdir;Unique=aaabcaUYqaaa;Perm=el; /sub
S> 226 That's all folks...
C> PASV
S> 227 Entering Passive Mode (127,0,0,1,255,42)
C> MLSD dir
S> 150 I tink I tee a trisector tree
D> Type=pdir;Unique=aaabcaUYqaaa;Perm=el; /sub
D> Type=pdir;Unique=aaabcaUYqaaa;Perm=el; ..
D> Type=file;Unique=aaab8cUYqaaa;Perm=r;Size=15039; mlst.c
D> Type=dir;Unique=aaabcfUYqaaa;Perm=el; ect
D> Type=cdir;Unique=aaabceUYqaaa;Perm=el; dir
D> Type=cdir;Unique=aaabceUYqaaa;Perm=el; /sub/dir
D> Type=dir;Unique=aaabchUYqaaa;Perm=el; misc
D> Type=file;Unique=aaab8bUYqaaa;Perm=r;Size=34589; ftpd.c
S> 226 That's all folks...
C> CWD dir/ect
S> 250 CWD command successful.
C> PWD
S> 257 "/sub/dir/ect" is current directory.
C> PASV
S> 227 Entering Passive Mode (127,0,0,1,255,40)
C> MLSD
S> 150 I tink I tee a trisector tree
D> Type=dir;Unique=aaabcgUYqaaa;Perm=el; ory
D> Type=pdir;Unique=aaabceUYqaaa;Perm=el; /sub/dir
D> Type=pdir;Unique=aaabceUYqaaa;Perm=el; ..
D> Type=cdir;Unique=aaabcfUYqaaa;Perm=el; /sub/dir/ect
S> 226 That's all folks...
C> CWD /files
S> 250 CWD command successful.
C> PASV
S> 227 Entering Passive Mode (127,0,0,1,255,36)
C> MLSD
S> 150 I tink I tee a trisector tree
D> Type=cdir;Unique=aaab8aUYqaaa;Perm=cpmel; /files
D> Type=pdir;Unique=aaaaacUYqaaa;Perm=cpmel; /
D> Type=pdir;Unique=aaaaacUYqaaa;Perm=cpmel; ..
D> Type=file;Unique=aaab8cUYqaaa;Perm=rf;Size=15039; mlst.c
D> Type=file;Unique=aaab8bUYqaaa;Perm=rf;Size=34589; ftpd.c
S> 226 That's all folks...



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C> RNFR mlst.c
S> 350 File exists, ready for destination name
C> RNTO list.c
S> 250 RNTO command successful.
C> PASV
S> 227 Entering Passive Mode (127,0,0,1,255,34)
C> MLSD
S> 150 I tink I tee a trisector tree
D> Type=file;Unique=aaab8cUYqaaa;Perm=rf;Size=15039; list.c
D> Type=pdir;Unique=aaaaacUYqaaa;Perm=cpmel; /
D> Type=pdir;Unique=aaaaacUYqaaa;Perm=cpmel; ..
D> Type=file;Unique=aaab8bUYqaaa;Perm=rf;Size=34589; ftpd.c
D> Type=cdir;Unique=aaab8aUYqaaa;Perm=cpmel; /files
S> 226 That's all folks...

   The server shown here returns its directory listings in seemingly
   random order, and even seems to modify the order of the directory as
   its contents change -- perhaps the underlying directory structure is
   based upon hashing of some kind.  Note that the "pdir" and "cdir"
   entries are interspersed with other entries in the directory.  Note
   also that this server does not show a "pdir" entry when listing the
   contents of the root directory of the virtual filestore; however, it
   does however include multiple "cdir" and "pdir" entries when it feels
   inclined.  The server also uses obnoxiously "cute" messages.

7.8.  FEAT Response for MLSx

   When responding to the FEAT command, a server-FTP process that
   supports MLST, and MLSD, plus internationalization of pathnames, MUST
   indicate that this support exists.  It does this by including a MLST
   feature line.  As well as indicating the basic support, the MLST
   feature line indicates which MLST facts are available from the
   server, and which of those will be returned if no subsequent "OPTS
   MLST" command is sent.

      mlst-feat     = SP "MLST" [SP factlist] CRLF
      factlist      = 1*( factname ["*"] ";" )

   The initial space shown in the mlst-feat response is that required by
   the FEAT command, two spaces are not permitted.  If no factlist is
   given, then the server-FTP process is indicating that it supports
   MLST, but implements no facts.  Only pathnames can be returned.  This
   would be a minimal MLST implementation, and useless for most
   practical purposes.  Where the factlist is present, the factnames
   included indicate the facts supported by the server.  Where the
   optional asterisk appears after a factname, that fact will be
   included in MLST format responses, until an "OPTS MLST" is given to
   alter the list of facts returned.  After that, subsequent FEAT



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   commands will return the asterisk to show the facts selected by the
   most recent "OPTS MLST".

   Note that there is no distinct FEAT output for MLSD.  The presence of
   the MLST feature indicates that both MLST and MLSD are supported.

7.8.1.  Examples

C> Feat
S> 211- Features supported
S>  REST STREAM
S>  MDTM
S>  SIZE
S>  TVFS
S>  UTF8
S>  MLST Type*;Size*;Modify*;Perm*;Unique*;UNIX.mode;UNIX.chgd;X.hidden;
S> 211 End

   Aside from some features irrelevant here, this server indicates that
   it supports MLST including several, but not all, standard facts, all
   of which it will send by default.  It also supports two OS dependent
   facts, and one locally defined fact.  The latter three must be
   requested expressly by the client for this server to supply them.

C> Feat
S> 211-Extensions supported:
S>  CLNT
S>  MDTM
S>  MLST type*;size*;modify*;UNIX.mode*;UNIX.owner;UNIX.group;unique;
S>  PASV
S>  REST STREAM
S>  SIZE
S>  TVFS
S>  Compliance Level: 19981201 (IETF mlst-05)
S> 211 End.

   Again, in addition to some irrelevant features here, this server
   indicates that it supports MLST, four of the standard facts, one of
   which ("unique") is not enabled by default, and several OS dependent
   facts, one of which is provided by the server by default.  This
   server actually supported more OS dependent facts.  Others were
   deleted for the purposes of this document to comply with document
   formatting restrictions.








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C> FEAT
S> 211-Features supported
S>  MDTM
S>  MLST Type*;Size*;Modify*;Perm;Unique*;
S>  REST STREAM
S>  SIZE
S>  TVFS
S> 211 End

   This server has wisely chosen not to implement any OS dependent
   facts.  At the time of writing this document, no such facts have been
   defined (using the mechanisms of section 10.1) so rational support
   for them would be difficult at best.  All but one of the facts
   supported by this server are enabled by default.

7.9.  OPTS Parameters for MLST

   For the MLSx commands, the Client-FTP may specify a list of facts it
   wishes to be returned in all subsequent MLSx commands until another
   OPTS MLST command is sent.  The format is specified by:

      mlst-opts     = "OPTS" SP "MLST"
                      [ SP 1*( factname ";" ) ]

   By sending the "OPTS MLST" command, the client requests the server to
   include only the facts listed as arguments to the command in
   subsequent output from MLSx commands.  Facts not included in the
   "OPTS MLST" command MUST NOT be returned by the server.  Facts that
   are included should be returned for each entry returned from the MLSx
   command where they meaningfully apply.  Facts requested that are not
   supported, or that are inappropriate to the file or directory being
   listed should simply be omitted from the MLSx output.  This is not an
   error.  Note that where no factname arguments are present, the client
   is requesting that only the file names be returned.  In this case,
   and in any other case where no facts are included in the result, the
   space that separates the fact names and their values from the file
   name is still required.  That is, the first character of the output
   line will be a space, (or two characters will be spaces when the line
   is returned over the control connection) and the file name will start
   immediately thereafter.

   Clients should note that generating values for some facts can be
   possible, but very expensive, for some servers.  It is generally
   acceptable to retrieve any of the facts that the server offers as its
   default set before any "OPTS MLST" command has been given, however
   clients should use particular caution before requesting any facts not
   in that set.  That is, while other facts may be available from the
   server, clients should refrain from requesting such facts unless



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   there is a particular operational requirement for that particular
   information, which ought be more significant than perhaps simply
   improving the information displayed to an end user.

   Note, there is no "OPTS MLSD" command, the fact names set with the
   "OPTS MLST" command apply to both MLST and MLSD commands.

   Servers are not required to accept "OPTS MLST" commands before
   authentication of the user-PI, but may choose to permit them.

7.9.1.  OPTS MLST Response

   The "response-message" from [6] to a successful OPTS MLST command has
   the following syntax.

      mlst-opt-resp = "MLST OPTS" [ SP 1*( factname ";" ) ]

   This defines the "response-message" as used in the "opts-good"
   message in RFC 2389 [6].

   The facts named in the response are those that the server will now
   include in MLST (and MLSD) response, after the processing of the
   "OPTS MLST" command.  Any facts from the request not supported by the
   server will be omitted from this response message.  If no facts will
   be included, the list of facts will be empty.  Note that the list of
   facts returned will be the same as those marked by a trailing
   asterisk ("*") in a subsequent FEAT command response.  There is no
   requirement that the order of the facts returned be the same as that
   in which they were requested, or that in which they will be listed in
   a FEAT command response, or that in which facts are returned in MLST
   responses.  The fixed string "MLST OPTS" in the response may be
   returned in any case, or mixture of cases.

7.9.2.  Examples

C> Feat
S> 211- Features supported
S>  MLST Type*;Size;Modify*;Perm;Unique;UNIX.mode;UNIX.chgd;X.hidden;
S> 211 End
C> OptS Mlst Type;UNIX.mode;Perm;
S> 200 MLST OPTS Type;Perm;UNIX.mode;
C> Feat
S> 211- Features supported
S>  MLST Type*;Size;Modify;Perm*;Unique;UNIX.mode*;UNIX.chgd;X.hidden;
S> 211 End
C> opts MLst lang;type;charset;create;
S> 200 MLST OPTS Type;
C> Feat



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S> 211- Features supported
S>  MLST Type*;Size;Modify;Perm;Unique;UNIX.mode;UNIX.chgd;X.hidden;
S> 211 End
C> OPTS mlst size;frogs;
S> 200 MLST OPTS Size;
C> Feat
S> 211- Features supported
S>  MLST Type;Size*;Modify;Perm;Unique;UNIX.mode;UNIX.chgd;X.hidden;
S> 211 End
C> opts MLst unique type;
S> 501 Invalid MLST options
C> Feat
S> 211- Features supported
S>  MLST Type;Size*;Modify;Perm;Unique;UNIX.mode;UNIX.chgd;X.hidden;
S> 211 End

   For the purposes of this example, features other than MLST have been
   deleted from the output to avoid clutter.  The example shows the
   initial default feature output for MLST.  The facts requested are
   then changed by the client.  The first change shows facts that are
   available from the server being selected.  Subsequent FEAT output
   shows the altered features as being returned.  The client then
   attempts to select some standard features that the server does not
   support.  This is not an error, however the server simply ignores the
   requests for unsupported features, as the FEAT output that follows
   shows.  Then, the client attempts to request a non-standard, and
   unsupported, feature.  The server ignores that, and selects only the
   supported features requested.  Lastly, the client sends a request
   containing a syntax error (spaces cannot appear in the factlist.)
   The server-FTP sends an error response and completely ignores the
   request, leaving the fact set selected as it had been previously.

   Note that in all cases, except the error response, the response lists
   the facts that have been selected.

C> Feat
S> 211- Features supported
S>  MLST Type*;Size*;Modify*;Perm*;Unique*;UNIX.mode;UNIX.chgd;X.hidden;
S> 211 End
C> Opts MLST
S> 200 MLST OPTS
C> Feat
S> 211- Features supported
S>  MLST Type;Size;Modify;Perm;Unique;UNIX.mode;UNIX.chgd;X.hidden;
S> 211 End
C> MLst tmp
S> 250- Listing tmp
S>   /tmp



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S> 250 End
C> OPTS mlst unique;size;
S> 200 MLST OPTS Size;Unique;
C>  MLst tmp
S> 250- Listing tmp
S>  Unique=keVO1+YZ5; /tmp
S> 250 End
C> OPTS mlst unique;type;modify;
S> 200 MLST OPTS Type;Modify;Unique;
C> MLst tmp
S> 250- Listing tmp
S>  Type=dir;Modify=19990930152225;Unique=keVO1+YZ5; /tmp
S> 250 End
C> OPTS mlst fish;cakes;
S> 200 MLST OPTS
C> MLst tmp
S> 250- Listing tmp
S>   /tmp
S> 250 End
C> OptS Mlst Modify;Unique;
S> 200 MLST OPTS Modify;Unique;
C> MLst tmp
S> 250- Listing tmp
S>  Modify=19990930152225;Unique=keVO1+YZ5; /tmp
S> 250 End
C> opts MLst fish cakes;
S> 501 Invalid MLST options
C> MLst tmp
S> 250- Listing tmp
S>  Modify=19990930152225;Unique=keVO1+YZ5; /tmp
S> 250 End

   This example shows the effect of changing the facts requested upon
   subsequent MLST commands.  Notice that a syntax error leaves the set
   of selected facts unchanged.  Also notice exactly two spaces
   preceding the pathname when no facts were selected, either
   deliberately, or because none of the facts requested were available.

8.  Impact on Other FTP Commands

   Along with the introduction of MLST, traditional FTP commands must be
   extended to allow for the use of more than US-ASCII [1] or EBCDIC
   character sets.  In general, the support of MLST requires support for
   arbitrary character sets wherever file names and directory names are
   allowed.  This applies equally to both arguments given to the
   following commands and to the replies from them, as appropriate.





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      APPE                                RMD
      CWD                                 RNFR
      DELE                                RNTO
      MKD                                 STAT
      PWD                                 STOR
      RETR                                STOU

   The arguments to all of these commands should be processed the same
   way that MLST commands and responses are processed with respect to
   handling embedded spaces, CRs and NULs.  See section 2.2.

9.  Character Sets and Internationalization

   FTP commands are protocol elements, and are always expressed in
   ASCII.  FTP responses are composed of the numeric code, which is a
   protocol element, and a message, which is often expected to convey
   information to the user.  It is not expected that users normally
   interact directly with the protocol elements, rather the user-FTP
   process constructs the commands, and interprets the results, in the
   manner best suited for the particular user.  Explanatory text in
   responses generally has no particular meaning to the protocol.  The
   numeric codes provide all necessary information.  Server-PIs are free
   to provide the text in any language that can be adequately
   represented in ASCII, or where an alternative language and
   representation has been negotiated (see [7]) in that language and
   representation.

   Pathnames are expected to be encoded in UTF-8 allowing essentially
   any character to be represented in a pathname.  Meaningful pathnames
   are defined by the server NVFS.

   No restrictions at all are placed upon the contents of files
   transferred using the FTP protocols.  Unless the "media-type" fact is
   provided in a MLSx response nor is any advice given here that would
   allow determining the content type.  That information is assumed to
   be obtained via other means.

10.  IANA Considerations

   This specification makes use of some lists of values currently
   maintained by the IANA, and creates two new lists for the IANA to
   maintain.  It does not add any values to any existing registries.

   The existing IANA registries used by this specification are modified
   using mechanisms specified elsewhere.






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10.1.  The OS-Specific Fact Registry

   A registry of OS specific fact names shall be maintained by the IANA.
   The OS names for the OS portion of the fact name must be taken from
   the IANA's list of registered OS names.  To add a fact name to this
   OS specific registry of OS specific facts, an applicant must send to
   the IANA a request, in which is specified the OS name, the OS
   specific fact name, a definition of the syntax of the fact value,
   which must conform to the syntax of a token as given in this
   document, and a specification of the semantics to be associated with
   the particular fact and its values.  Upon receipt of such an
   application, and if the combination of OS name and OS specific fact
   name has not been previously defined, the IANA will add the
   specification to the registry.

   Any examples of OS specific facts found in this document are to be
   treated as examples of possible OS specific facts, and do not form a
   part of the IANA's registry merely because of being included in this
   document.

10.2.  The OS-Specific Filetype Registry

   A registry of OS specific file types shall be maintained by the IANA.
   The OS names for the OS portion of the fact name must be taken from
   the IANA's list of registered OS names.  To add a file type to this
   OS specific registry of OS specific file types, an applicant must
   send to the IANA a request, in which is specified the OS name, the OS
   specific file type, a definition of the syntax of the fact value,
   which must conform to the syntax of a token as given in this
   document, and a specification of the semantics to be associated with
   the particular fact and its values.  Upon receipt of such an
   application, and if the combination of OS name and OS specific file
   type has not been previously defined, the IANA will add the
   specification to the registry.

   Any examples of OS specific file types found in this document are to
   be treated as potential OS specific file types only, and do not form
   a part of the IANA's registry merely because of being included in
   this document.












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11.  Security Considerations

   This memo does not directly concern security.  It is not believed
   that any of the mechanisms documented here impact in any particular
   way upon the security of FTP.

   Implementing the SIZE command, and perhaps some of the facts of the
   MLSx commands, may impose a considerable load on the server, which
   could lead to denial of service attacks.  Servers have, however,
   implemented this for many years, without significant reported
   difficulties.

   The server-FTP should take care not to reveal sensitive information
   about files to unauthorised parties.  In particular, some underlying
   filesystems provide a file identifier that, if known, can allow many
   of the filesystem protection mechanisms to be by-passed.  That
   identifier would not be a suitable choice to use as the basis of the
   value of the unique fact.

   The FEAT and OPTS commands may be issued before the FTP
   authentication has occurred [6].  This allows unauthenticated clients
   to determine which of the features defined here are supported, and to
   negotiate the fact list for MLSx output.  No actual MLSx commands may
   be issued however, and no problems with permitting the selection of
   the format prior to authentication are foreseen.

   A general discussion of issues related to the security of FTP can be
   found in [13].























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12.  Normative References

   [1]  Coded Character Set--7-bit American Standard Code for
        Information Interchange, ANSI X3.4-1986.

   [2]  Yergeau, F., "UTF-8, a transformation format of ISO 10646", RFC
        3629, November 2003.

   [3]  Postel, J. and J. Reynolds, "File Transfer Protocol (FTP)", STD
        9, RFC 959, October 1985.

   [4]  Bradner, S., "Key words for use in RFCs to Indicate Requirement
        Levels", BCP 14, RFC 2119, March 1997.

   [5]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
        Specifications: ABNF", RFC 4234, October 2005.

   [6]  Hethmon, P. and R. Elz, "Feature negotiation mechanism for the
        File Transfer Protocol", RFC 2389, August 1998.

   [7]  Curtin, B., "Internationalization of the File Transfer
        Protocol", RFC 2640, July 1999.

   [8]  Postel, J. and J. Reynolds, "Telnet protocol Specification", STD
        8, RFC 854, May 1983.

   [9]  Braden, R,. "Requirements for Internet Hosts -- Application and
        Support", STD 3, RFC 1123, October 1989.

   [10] ISO/IEC 10646-1:1993  "Universal multiple-octet coded character
        set (UCS) -- Part 1: Architecture and basic multilingual plane",
        International Standard -- Information Technology, 1993.

   [11] Internet Assigned Numbers Authority.  http://www.iana.org
        Email: iana@iana.org.

   [12] Phillips, A. and M. Davis, "Tags for Identifying Languages", BCP
        47, RFC 4646, September 2006.

   [13] Allman, M. and S. Ostermann, "FTP Security Considerations" RFC
        2577, May 1999.










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Acknowledgments

   This document is a product of the FTPEXT working group of the IETF.

   The following people are among those who have contributed to this
   document:

      Alex Belits
      D. J. Bernstein
      Dave Cridland
      Martin J. Duerst
      Bill Fenner (and the rest of the IESG)
      Paul Ford-Hutchinson
      Mike Gleason
      Mark Harris
      Stephen Head
      Alun Jones
      Andrew Main
      James Matthews
      Luke Mewburn
      Jan Mikkelsen
      Keith Moore
      Buz Owen
      Mark Symons
      Stephen Tihor
      and the entire FTPEXT working group of the IETF.

   Apologies are offered to any inadvertently omitted.

   The description of the modifications to the REST command and the MDTM
   and SIZE commands comes from a set of modifications suggested for STD
   9, RFC 959 by Rick Adams in 1989.  A document containing just those
   commands, edited by David Borman, has been merged with this document.

   Mike Gleason, Alun Jones and Luke Mewburn provided access to FTP
   servers used in some of the examples.

   All of the examples in this document are taken from actual
   client/server exchanges, though some have been edited for brevity, or
   to meet document formatting requirements.

RFC Editor Note:

   Several of the examples in this document exceed the RFC standard line
   length of 72 characters.  Since this document is a standards-track
   result of an IETF working group and is important to an IETF sub-
   community, the RFC Editor is publishing it with the margin
   violations.  This is not a precedent.



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Author's Address

   Paul Hethmon
   Hethmon Software
   10420 Jackson Oaks Way, Suite 201
   Knoxville, TN 37922

   EMail: phethmon@hethmon.com











































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Full Copyright Statement

   Copyright (C) The IETF Trust (2007).

   This document is subject to the rights, licenses and restrictions
   contained in BCP 78, and except as set forth therein, the authors
   retain all their rights.

   This document and the information contained herein are provided on an
   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND
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   The IETF takes no position regarding the validity or scope of any
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   this document or the extent to which any license under such rights
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   Copies of IPR disclosures made to the IETF Secretariat and any
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   attempt made to obtain a general license or permission for the use of
   such proprietary rights by implementers or users of this
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   The IETF invites any interested party to bring to its attention any
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Acknowledgement

   Funding for the RFC Editor function is currently provided by the
   Internet Society.







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